The term "wait for event" refers to a type of step that can be incorporated into your drip campaign.
This step allows you to wait for a recipient to take a certain action before proceeding to the next step in the drip campaign. Using this step creates an interactive, or real time, experience for the recipient.
NOTE: You can always set a max wait time for a Wait for Event step. Say if after two weeks you don't want to wait for the subscriber action any more, you would choose two weeks as your max time.