An invite link is a link that you create and use to invite someone to a specific server.
Creating an Invite Link
Keep in mind that, whatever channel you are in, the channel you are in will be where the invite link sends the person who clicks the link.
To create and invite link on PC, go to the top left side of your screen while in the server you want to invite someone to. Click the title of the server. Then, the second bar should say "Invite People". Click that, and create the link!
Changing the Expiration Time or Number of Uses
To change the expiration time or the number of uses, do the same steps as creating the link, but go to the bottom and click "Edit invite link". Then, there should pop up a menu that allows you to change these things.