The pre-dispatch gate, and what it stops
The final checks a flight passes before it is released, and why they run again at dispatch.
Last reviewed
The pre-dispatch gate is the last check before a flight is released. It re-runs the booking's checks against the state right now, so anything that changed since the booking was made is caught at the desk rather than at the aircraft.
What it checks
A booking was valid when it was made, but the world moves between then and the flight: a snag is raised, a medical lapses, a duty limit tightens. The gate re-evaluates the people, the aircraft, maintenance, currency and your school's rules as they stand at this moment. If something no longer holds, the flight is held with the reason.
The pilot-in-command acknowledgement
Where your school requires it, the person in command acknowledges the flight before release. That acknowledgement is recorded against the flight, so the chain of who released what is clear afterwards.
Why it runs twice
The schedule already checks a booking for the time of the flight when it is made. The gate checks again at the moment of dispatch, because that is the only point at which "now" and "the flight" are the same. Passing the gate is what turns a checked-in flight into a dispatched one. See Why was a booking blocked? for how to read a held flight.