The seriousness of a trespassing charge depends on where you were trespassing and if you've violated a trespassing warning or it was just a trespass warning.
If you just happen to trespass on someone's property and did no damage to the property then most times when the cops are called the person will just get a trespass warning.
The cop may have you sign a paper saying that you are gonna stay off the property and will not trespass again.
Then if you trespass again after the trespass warning and the cops are called again then you will get arrested.
Usually the trespass charge is just a misdemeanor charge which means you'll likely be able to bail out of jail for $500.00 or less.
Then you show up at court and will likely just get a fine or nothing else may happen.
But if you trespass on federal property then it can become a felony especially if you've been warned previously.
Then you could go to prison for a year or more.
But most trespassing charges are simple misdemeanors and nothing much usually happens.