It's the "chunks of space" allocated for that particular file. Have a look at Block_(data_storage).

Want to find out more? Here is an excerpt from Suffering from Buffering:

... The blocks on your disk are probably about 8K bytes, and your computer hardware is probably designed to transfer an entire block of data from the disk at once. So instead of asking the system for a little bit of text, Perl actually asks the system for an entire blockful, which hardly takes longer to get than a little bit would have. ...