Hadn't is a contraction of “had not,” combining the two words into a shorter form. When you say hadn't, you're describing something that didn't happen before another event in the past.

For example, if you arrived at school and realized you hadn't brought your lunch, the forgetting happened before the arriving. If a team lost a game because they hadn't practiced enough, the lack of practice came before the loss. You might say, “I hadn't seen that movie before last night,” meaning up until last night, you'd never watched it.

The word helps you talk about the order of past events. Compare “I didn't study” (simple past) with “I hadn't studied” (something that didn't happen before another past moment). When you use hadn't, you're usually explaining why something else happened or didn't happen: “She hadn't eaten breakfast, so she felt hungry by lunchtime.”

Like other contractions such as can't or won't, hadn't is perfectly acceptable in everyday speech and informal writing, though in very formal writing, some people prefer writing out “had not” in full.