A sec is a very short time, just a moment.
The word sec is used when you want to say that something will happen very quickly.
Examples:
I will be back in a sec.
Wait a sec, I need to tie my shoe.
I need to finish this in a sec.
I'll be there in a sec, just wait for me!
I can run to the store and back in a sec.
I'll be done in a sec, I promise.