A high-focus community might be one that deals with the minutiae of a particular rock band and its music. Straying too far from the intended topic — discussing, say, deconstruction of 19th century painters — would be discouraged.
High focus might be due to the original, continuing, needs of the community (perhaps derived from a single founder).
A low-focus community would allow a person to talk about anything. Of course, the problem there is that it takes at least two to have a discussion! The Internet is awash with still-born communities that did not gel due to a lack of focused intent, and hence a lack of participants attracted by a particular interest.