Advantage
Advantage is an resource that adds a d6 to the relevant roll, be it for Skills or Combat. It can be gained from being permanent additions to Skills by Levelling Up, from certain Status Effects, eating specific meals, or having certain Equipment. Advantage can also be given depending on the context surrounding whatever's being rolled: For instance, you may need to make a Reasoning check to solve a riddle, but you could be given an Advantage if you overheard the riddle beforehand. Multiple Advantages can be stacked at once, if they apply to the same roll. Making a Dexterity roll after eating a Healthy meal and having one Advantage checked on your Character Sheet mean you have an additional 2d6 to your roll.
The inverse to Advantage is Disadvantage, which removes a d6 from the relevant roll. Like Advantages, Disadvantages can stack, removing multiple dice from a roll if possible.
If an Advantage and a Disadvantage are applied to the same roll, they are cancelled out. Think of it like combining a positive and negative number of equal value to get 0.