So, you know how when you wear a shirt or a dress and it kind of dips down in the middle and shows some of your chest area? That is called a neckline. Cleavage is when that neckline dips down a little bit lower and shows more of your chest area - specifically the area between your two boobs.
When people talk about cleavage, they are usually talking about how it looks when a woman is wearing a certain type of clothing that pushes her boobs together to make them look bigger and closer together. This can be achieved with things like push-up bras, padded bras, or even just adjusting the straps of a regular bra.
Cleavage can be seen as attractive to some people, but it's also important to remember that everyone's body is different and there is no "right" or "wrong" way to look. It's important to be happy and comfortable in your own skin, no matter what your cleavage looks like!