What is Boost?
Boost is a field on an Advertisement that will do just like its name implies, it will boost the likelihood that the Ad is displayed. By increasing the number on the boost, that Ad will gain greater weight in the distribution of Ads within the AdGroup making it more likely to be displayed. The boosting effect is relative to the other Ads in the AdGroup, however, and as such, if you increase the boost level of all Ads in the group, it will have less of an effect as the boosts can offset each other.
How does it work?
To increase the likelihood that one Ad will be shown over another Ad in an AdGroup, you can increase the boost number. Oppositely, if you wish to decrease the likelihood the Ad will be shown, you can decrease the value.
