A Choice Block is started at a Choice (XOR) Split and ended at a Choice (XOR) Join (the 'brackets'). It indicates that, of the possible paths within the block, no more than one can be followed.
The split has several transitions outbound from it, indicating the possible paths that a process instance could follow. Since this is a Choice block, the paths are mutually exclusive - only one will be followed by a given process instance.
The Choice Split must be matched by a corresponding Choice Join. This indicates the point at which the process ceases to be distinct for each path, so the paths are merged back together (i.e. the remaining process is common).