Sexual harassment is when someone makes you feel uncomfortable or scared by doing things related to sex or your body that you don't want or didn't ask for.
For example, if someone touches your private parts when you don't want them to, says things to make you feel bad or ashamed about your body, or makes sexual jokes or comments to you all the time even though you tell them to stop, that can be sexual harassment.
It's really important to say no and tell a grown-up you trust if someone is doing things that make you feel uncomfortable or unsafe. No one has the right to touch or say things to you that you don't want, and it's not your fault if someone does.