Trespass is like going into your neighbor's house or backyard without asking. It's not okay, because you could get hurt or something could happen to their things.
Just like how you need permission to go into someone's house, you also need permission to go onto someone else's land. If you don't have permission and you go onto someone else's land, that's called trespassing.
It's important to remember that when you're on someone else's land, you need to be careful and respectful of their things. And, if someone tells you to leave their land or property, you should listen to them and go back to your own land.