just codes you can give to members, instead of you having to look all about the carribean for them, you can just give them a code.
As Ofx said, they can be either single, multiple, or unlimited
Single= only one person can use that code (NOT RECCOMENDED FOR RECRUITING) after it is used once, then you must make a new code
Multiple= you can select how many times that your code can be used
Unlimited= that code can be used FOREVER! untill you remove it
to get too all of this, you must be an officer or a GM
Go to Guild options
>>invatation options
then you make your code on how you like