The delay after queue option will only appear when you have an existing delay in your Zap, in order to prevent race conditions and rate limiting. It can also be used in any scenario where a Zap or multiple Zaps may trigger many tasks in parallel and you want to force them to run in series.
Since these delays will queue up, the maximum number of tasks in the queue depends on the Time Delayed For value. For example, if you set it to one day, the maximum tasks in the queue will be 31, which means that the 32nd task created within that period will error on the delay step because the scheduled resume is too far away.
The action is not a lock that can guarantee that the steps following it will never run simultaneously. Slowdowns in our infrastructure and auto or manual replays after errors may cause steps to still run at the same time.