I'd is a contraction, which means it's a shortened form of two words squeezed together. Specifically, it combines “I” and “would” (as in I'd love to go to the movie) or “I” and “had” (as in I'd already finished my homework when you called).

The apostrophe shows where letters were removed when the two words merged together. People use contractions like this all the time in everyday speech and informal writing because they sound more natural and conversational than saying “I would” or “I had” every time.

You can usually tell which meaning fits by looking at what comes after: I'd like means “I would like,” while I'd eaten means “I had eaten.” This happens naturally when you read or speak: your brain figures out the right meaning from context without you even thinking about it.

Other common contractions work the same way: “you'd” (you would/you had), “he'd” (he would/he had), and “she'd” (she would/she had). Writers use contractions to make dialogue sound realistic, since that's how people actually talk.