A homogenous mixture has a uniform composition. This means it is made of two or more things, but the different things are spread out evenly.
A heterogenous mixture does not have a uniform composition. Things are randomly spread out.
You can often tell by looking. If something looks the same throughout the whole thing it is homogenous while if it looks different throughout it is heterogenous. For example, Gatorade is homogenous because it looks the same throughout the entire bottle.