An add-on is an additional buy in for an MTT. Similar to rebuys, the add-on is an additional amount of chips that can be bought at a set point in the tournament (during the add-on period).
The fundamental difference between add-ons and re-buys is that you can choose to add-on in an add-on poker tournament regardless of how many chips you have at that stage. Add-ons can usually be done until the end of the first break.
EXAMPLE “Every time I have been in a tournament that offers add-ons, I have been crippled shortly after the add-on period.”
James Hartigan runs through the rules to re-buys and add-ons in the video below.