"Had" is the past tense form of the verb "have." It is used to indicate possession, experiences, or actions that occurred in the past. For example, one might say, "I had a bicycle when I was a child," which shows ownership in the past. In addition to possession, "had" is often used in various grammatical structures, such as the past perfect tense. This tense describes an action that was completed before another action in the past. For instance, "She had finished her homework before dinner" illustrates that the homework was completed prior to another event, dinner.