Had is the past tense and past participle of have.

If you have been had, someone has tricked you, for example by selling you something at too high a price.

If you say that someone has had it, you mean they are in very serious trouble or have no hope of succeeding.

If you say that you have had it, you mean that you are very tired of something or very annoyed about it, and do not want to continue doing it or it to continue happening.

Had is sometimes used instead of 'if' to begin a clause which refers to a situation that might have happened but did not. For example, the clause 'had he been elected' means the same as 'if he had been elected'.