Upon resolution, this gives the targeted creature trample and creates a delayed-triggered ability that will trigger when the creature becomes blocked. The creature will get +1/+1 for each creature blocking it as that ability resolves. This means that if Barreling Attack is cast after blockers are declared, the targeted creature will gain trample but won’t be able to get +1/+1 because it’s too late for the ability to trigger (unless there is another combat phase in the turn).