When someone is trespassing they are basically just going on private property that they should not be on and do not have permission to be on the private property.
If you're trespassing and the cops are called the cops will warn you with a trespass warning and they will usually have you sign a form saying that you will not return to the property.
Then if you do return to the property after being warned then it can lead to an arrest where you can serve 90 days to a year in jail and pay a fine.
It can also be turned into a criminal trespassing charge as well.
But mostly when you get charged with a criminal trespassing charge you are usually both trespassing and have entered a building or home without permission.