What is the meaning about "xclip -sel"?

what is the meaning of 'xclip -sel clip'?

I can't find the '-sel' in xclip API.

xclip -sel clip < ~/.ssh/id_rsa.pub 

man xclip is your friend: https://linux.die.net/man/1/xclip

TL;DR it tells you which X selection to copy the text to