Add-on is a poker term that refers to the additional amount of chips players can add to their bankroll during a tournament, and allows you to get more chips than you received with your original buy-in.
There are two types of add-ons: an instant add-on and a final add-on. However, keep in mind that this greatly depends on the tournament you are participating in. Furthermore, another thing to note is that the add-on is not the same as the rebuy or re-entry.
For example, while you can rebuy only when you are low on chips and re-entry only when your stack is completely busted, you can add-on chips whenever you feel like, as long as it is in line with the rules laid out by the tournament organisers.
Tournaments often allow you to add-on chips until a specific point in the tournament, such as the fifth or tenth round, for example, so make sure to check the tournament rules if you wish to take advantage of the add-on option.