What is had?

Had is the simple past tense and past participle form of the verb "have". It is used in English grammar as an auxiliary verb to indicate possession, experience, obligation, or action in the past. For example, "I had a headache yesterday" indicates that at some point in the past, the speaker experienced a headache. "He had to leave early" indicates an obligation that existed in the past. Had can also be used as the main verb in some contexts, such as "I had a lot of fun at the party" or "She had a difficult childhood". Additionally, "had" can also function as a linking verb to connect the subject of a sentence to a noun or adjective, such as in "I had a feeling the movie would be good".