It'd is a contraction, a shortened form of two words squeezed together: “it would” or “it had.”

When you write it'd, you're saving time and making your writing sound more like natural speech. Instead of saying “It would be fun to visit the Grand Canyon,” you might say “It'd be fun to visit the Grand Canyon.” Instead of “It had been raining all morning,” you could write “It'd been raining all morning.”

Contractions like this work by replacing some letters with an apostrophe. The apostrophe shows where letters have been removed: the ould from “would” or the a from “had” disappear, and the apostrophe takes their place.

You'll see it'd more often in casual writing, dialogue, or informal emails than in formal essays or reports. When a character in a story says “It'd be great if we could leave early,” it sounds natural and conversational. But in a science report, you'd probably write out the full words: “It would be beneficial to begin the experiment earlier.”

One tricky thing: because it'd can mean either “it would” or “it had,” you need to rely on context to know which one the writer intended.