What does "1K-blocks" column mean in the output of `df`?

What does "1K-blocks" column mean in the output of df?

$ df
Filesystem     1K-blocks      Used Available Use% Mounted on
/dev/sda7       21181308  19302672    802668  97% /
udev             4070176         4   4070172   1% /dev
tmpfs             815536       972    814564   1% /run

I guess it is the size of a partition in KB?

Does "1K-blocks" mean the size of each block of a partition is 1KB?

Does a "block" here mean the same as a cluster of a file system?