An harass is to bother someone repeatedly or annoy them with unwanted attention.
When people say that they've been harassed by someone, it usually means that this person has been making their life difficult, maybe even making them feel uncomfortable or scared. This can happen online, when someone keeps sending mean messages or emails, or in person, when someone keeps following you around or won't leave you alone. Harassment can also be at work, where a boss or coworker makes unwanted comments or touches that make the other person feel uneasy. It's like being constantly nagged or pestered, and it can be really stressful for the person on the receiving end.
To annoy (someone) frequently or systematically; to pester.
