Bandwidth, in personal communications, refers to the amount of information that can be passed between individuals. This kind of bandwidth is different between different pairs of people. Some people you can communicate a lot with, others very little, even though you all speak the same language.
In this case, you can't really increase the raw data stream. All of us are communicating through words and gestures. Instead, you have to make these tokens carry more meaning. This happens when people know each other well and have common experiences to draw on.
The Coen Brothers, the sibling filmmakers, are known as the "two-headed director" to actors because if you ask either of them a question about the film, they'll give you the same answer. They can be said to have a very high bandwidth between them. However they are communicating between each other, it is obviously with a very economical use of words, based on their shared past experience.
When you communicate with a person on the street who you just met, it is usually a low-bandwidth connection. You can talk about the weather and recent news stories but little else.
We hesitate to talk about meaningful things with strangers because we fear they might not understand us or, worse, that they might be offended. You want to feel them out before you attempt complicated topics.
If you see that someone is reading a book that you have also read, then suddenly you have a body of knowledge in common. You can safely launch into a complicated discussion about the topic of the book, and the amount of information passed between you can be quite large relative to the number of words you are using. A person sitting near you who hasn't read the book might not have a clue what you are talking about, because he doesn't have the common knowledge base.
Since we are all operating with the same language—English—with a limited number of words at our disposal, bandwidth has to do with encoding and decoding mechanisms. When we have shared knowledge base with someone else, we can encode information into smaller packages, knowing that they will accurately decode the message at the other end.
But bandwidth involves more than shared knowledge. There are plenty of people who have years of shared experience with each other but who nonetheless communicate very poorly. We see them bickering in public places. No doubt, you too have been involved in relationships where there was shared experience but poor communication. It can be hell!
It turns out, high bandwidth involves a certain emotional sync between people. Furthermore, some people have such emotional blocks that they can't communicate deeply with anyone at all.
People develop emotional blocks when they become heavily invested in things—be they life choices or belief systems. People can also get out of sync with themselves, where there own brain is rife with inconsistencies. Both of these things can form a barrier to external communication.
Theoretically, the more common experience you have with someone, the better your communication will be, but it doesn't always work that way. Sometimes, communication just gets worse and worse, until you tiptoeing around, doing your best not to set them off.
If you are only communicating a small portion of what you think and feel to another person, then you have a low-bandwidth connection, regardless of how many years you have spent together. Like it or not, your investment in shared experience really hasn't paid off. Now, you have to decide whether the investment should continue.
The benefits of high bandwidth are great, especially in creative partnerships like the Coens'. You can economically communicate instructions to each other knowing they will be understood. The more experience you have together, the more complex projects you can collaborate on, like whole movies.
In a low-bandwidth relationship, you are forever trapped in a cycle of retractions and corrections, misinterpretations and apologies, and the volume of information you actually convey is very small. When you are out of emotional sync with your communication partner, nothing works right. Messages are decoded in a completely differently than how you encoded them, then you are supposed to apologize for how your message was misinterpreted.
If you're already heavily invested in the relationship, you may try all sorts of tricks and therapy to try to make communication better, but they rarely work. High bandwidth involves many personality factors, and for the most part, personalities can't be changed.
If you want high bandwidth, you have to build it, but you also have to select for it. Face it, you just can't communicate with most people in the world. You can discuss weather, sports and other non-controversial topics, but anything deeper is going is going to trigger their sensitivities. There's nothing you can do to change this.
While common experience is useful, there are also people you can communicate with almost instantly. 99 out of 100 people you sit beside in airplanes are dolts, but 1 out of 100 is reachable. You will get a lot farther investing in them then you will with the others.
Invest in a dolt and years later they will still be a dolt. And you will be watching your words very, very carefully.