An insult is a description of someone that will offend them. It may or may not be true. It is called derogatory language. Terms like foolish, stupid, idiot and moron are insults, because they say that a person's mind is not quick or smart.
Insulting someone's mother directly is a serious insult in many cultures.
Ritual insults are part of many cultures. For example, they can be found in sports and military training. They are also very common in jargons. For example, the word newbie is a part of net jargon. Calling someone a newbie is usually insulting.
One should be very careful when using new words to describe others.
Usually, someone insults others because they want to feel like they are better or have more power (influence) than the people they insult. They may want this because they are actually afraid that they are worse or less powerful than the people they are insulting.
When someone is insulted, their pride is hurt. They may want to fight back by insulting the person who insulted them, or by telling someone who is older.