7 Minutes of Terror
It takes seven minutes for a vehicles to descend through the Martian atmosphere to the surface. The distance between Earth and Mars is always greater than seven light minutes when these landings take place, so by the time Earth gets the message that the vehicles has made contact with the atmosphere, it is already on the ground. Hopefully in one piece.
This means the vehicle must perform every part of the landing sequence in sequence successfully. If any part of this sequence fails, the vehicle will crash onto the surface. For Perseverance, things are particularly tough. It is attempting to land in a crater filled with hazards such as cliffs, rocks, boulders, smaller craters, dunes (which may or may not be petrified) and so on. The following video is a short, fascinating description of everything it has to do to make it to the surface safely. There is an articles on the specifics of the navigational landing system on this page.