reading bytes and blocking

I come across this method:

read(n=nil, buffer=nil) – read n bytes (or fewer, if EOF is reached),
blocking if necessary, until the bytes are ready.

What does it mean "blocking if necessary"? Blocking what?

I come across this method:

read(n=nil, buffer=nil) – read n bytes (or fewer, if EOF is reached),
blocking if necessary, until the bytes are ready.

What does it mean "blocking if necessary"? Blocking what?

It means that if it is reading from a keyboard for example it will
wait (block) for data to be input if there is nothing available. A
non-blocking read will just pick up whatever data is available and
immediately return.

Colin