There's is a contraction, which means it's two words squeezed together into one shorter word. Specifically, there's combines “there” and “is” into a single word, with an apostrophe taking the place of the missing letters.

You use there's when you want to point out that something exists or is happening. “There's a package at the door” means a package is at the door. “There's no school tomorrow” means there is no school tomorrow. “There's something I need to tell you” introduces information you're about to share.

The apostrophe in there's marks where the letter “i” from “is” was removed. This kind of shortening happens naturally in conversation because it's faster and feels more casual than saying “there is” every time.

Be careful not to confuse there's with other words that sound similar. Theirs (without an apostrophe) means something belongs to them, like “That backpack is theirs.” They're means “they are,” as in “They're going to the park.”

One grammar note: technically, there's should only be used with singular things (“there's one problem”) rather than plural things (“there are two problems”), though you'll hear people use it both ways in casual conversation.