これ (kore) means "this." It refers to an object that is close to the speaker. If you're the speaker talking about an object near you, you would use これ.
それ (sore) means "that," but you use it when the object is close to the person you are talking to. Imagine if you were buying something and the object was behind the counter near the cashier. If you were to point out something that you wanted that was near the cashier, you would use それ (and they would use これ because it is near them).
あれ (are) refers to an object that is far from you and the person you are talking to. So if you pointed to something out at night in the sky and told someone to look at it, you would use あれ. If the person you were talking to is speaking about that same thing, they would use あれ also.
Another word that can be related to these words is どれ (dore), meaning "which." It is a question word. You use どれ to specify an object. Some question words like どれ and なに cannot be followed by the particle は, and must use the particle が (ga). We'll learn more about that later.
どれがやすいですか。 (dore ga yasui desu ka?)
Which one is cheap?
Together, these set of words are called "ko-so-a-do." Take the first characters of これ, それ, あれ, and どれ, and you get こそあど (ko-so-a-do). There are more words similar to these that are in the set of ko-so-a-do, and we'll learn them next.