A block is a section of a buffer, you mark it by specifying its edges (i.e. the first and last characters). This part of the buffer can then have various things done to it, for example insert it somewhere else.
Each window can only have a single block marked at any one time, it will be displayed in the reverse of normal text (i.e. white on black, not black on white).
Go to the first, previous, next, last section, table of contents.