This is one of the most common mistakes in the English language. We have a lot of words that sound the same and look the same, but have totally different meanings. Most of them also have different spellings, which makes them a nightmare to differentiate. Common mistakes are between:
“There” and “Their” (One is to pinpoint a specific place, and the other is a possessive for a group of people – something that belongs to them).
“Too” and “To” (One is used to add something to a point, be it a person or a fact, and the other is used before a location).
and “Your” and “You’re”. (One is the possessive, something belongs to you, and the other is a contraction, meaning “You are”).
I’ve made some of those mistakes as a student, but I was always able to correct them before anyone else saw them. It’s very easy to make those mistakes, especially with “To” and “Too” – just don’t let your finger rest on the “O” key too long! It’s also something spellcheck can’t see, as you’ve spelled the word correctly – it’s just not the word you wanted. But of course, don’t worry too much. You know the spelling. Just give your work a good check to make sure no mistaken homonyms have crept in. They’re just as easy to fix, as they are to make.