The abbreviation i.e. means “in other words” or “that is to say.” Writers use it to restate or clarify something they just wrote in a different way that might be easier to understand.

i.e. comes from the Latin phrase id est, which means “that is.” When you see i.e. in a sentence, what follows is another way of saying the same thing. For example: “I love citrus fruits, i.e., oranges, lemons, and grapefruits.” Here, “citrus fruits” and the list that follows are meant to refer to the same thing.

People often confuse i.e. with e.g., but they're different. Use i.e. when you're rephrasing or clarifying. Use e.g. when you're giving examples. If you write “I need a writing tool, i.e., a pencil,” you mean specifically a pencil. If you write “I need a writing tool, e.g., a pencil,” you mean a pencil is one example among other possibilities, like pens or markers.

You'll see i.e. in formal writing, textbooks, and instructions. In casual conversation, people usually just say “in other words” or “that is” instead.