There are two kinds of boosters: XP and chips boosters. The boosters provide extra chips or extra XP. The additional chips will be added to your total number of chips and not to the chips that you have on the table. Boosters come in two sizes: S and L, the S being the smallest bonus and the L being the largest bonus you can receive.

Boosters you have purchased are activated automatically.
The boosters have a power of 100% when they are activated, and this will gradually reduce after each win, which means that a booster’s effect can be used up before a certain period of time elapses. The reduction in power is calculated based on the amount of the chips or XP won.

If you click on the question mark next to the boosters it states:
"They are valid for 7 days, but depending on the way you are playing the could be used up before they expire."

Important hint: The boosters do not influence the cards at the table!