They'd is a contraction, which means it's two words squeezed together into one shorter form. It combines “they” and “would” or “they” and “had,” with an apostrophe marking where letters got left out.

Most often, they'd means “they would.” If your friends say “they'd love to come to your party,” they mean “they would love to come.” When you write “they'd be surprised,” you're saying “they would be surprised.” This version shows up when you're talking about what might happen or what someone might do.

Sometimes they'd means “they had,” usually right before another verb. “They'd already finished their homework” means “they had already finished.” “They'd never seen anything like it” means “they had never seen anything like it.” This version helps you talk about things that happened in the past before something else happened.

In speech, both meanings sound identical, so you figure out which one makes sense from context. The “they would” meaning is far more common in everyday conversation. Like other contractions (don't, can't, we'll), they'd makes writing feel more natural and conversational, the way people actually talk.