The wait time is the interval between the recipient receiving one message and the recipient being sent the next one.
You can set the interval time in days, hours and minutes.
For example your Call to Action is a webinar registration. The campaign goal is defined by recipients being in a specific group. You set the wait time between the first and second promo messages to 7 days. The recipient receives the first message. 7 days later the campaign checks IF the recipient is in the group (meaning they reached the goal of registering for the webinar) and IF NOT then the campaign sends the second promo message.
Transition recipient to Campaign End Stage
You also set the time interval between the recipient receiving the LAST message and the end of the campaign. This can be considered to be a grace period. For example, the last message is sent to the recipient. They then have a grace period of 10 days (which you set) to reach the goal of registering for the webinar. Remember, no matter when in the campaign (EVEN if the campaign is over), if a recipient reaches the goal, they will be moved to this stage.