Once you’ve stopped the car, you want it to stay exactly where you put it. Securing it means gravity won’t make it roll away, nor will it move if someone else pushes into it.
Secure it using  the handbrake and  taking it out of gear (= selecting neutral, so your own engine can’t accidentally pull you forward).
It’s like a cyclist who puts their feet down once stopped. Or like a ship that doesn’t want to drift at sea, due of wind or tides : it lowers an anchor.