It is important that visitors are not given the opportunity to start an engagement when agents with a necessary skill are not available, or already servicing chats. A skill can be provided with a max-wait-time. An engagement will not be displayed when a vistor is targetted by a campaign if their estimated-wait-time is higher than the max-wait-time of the skill. This is to ensure that visitors do not click an engagement and end up waiting for a long time to be answered by an agent.
Having said this, you may wish to override this setting for a particular engagement, and thus allow the engagement to be displayed to a visitor regardless of their calculated-wait-time. In the Engagement Studio, the option: "Ignore max wait time" means that the engagement will be displayed and visitors will be able to click and wait for an agent, as long as at least one agent is online.