"Preps" is short for "preparations". "Prepping" is short for "preparing". What am I preparing my preparations for? Emergencies.

Let's say something happens and I can't get to the store to buy food. I have enough food to last quite a while at my house.
What if the town's water system breaks down? I have drinkable water, along with some just for washing in.
If the power goes out I have flashlights and kerosene lanterns in every room of the house. As well as lighters for lighting the kerosene lanterns. I have batteries, and adapters so I can use AAA and AA size batteries in things that are made for larger sizes.

You get the idea, I hope. Make sure you are prepared to get through any emergency situation that could come up, without too much trouble. Different areas have different trouble that can arise (earthquakes, tsunamis, wildfires, riots, tornadoes, etc.), so know what can happen where you live and make yourself ready.