To oxidize is to chemically combine with oxygen. If you have a rusty car, that is the result of "oxidizing."
In chemistry class, you'll probably learn about oxidizing, which is when oxygen molecules combine with other molecules to produce an oxide. The most common example of oxidizing is rusting: oxygen interacts with iron molecules to produce the new (and unwelcome) iron oxide molecules, better known as rust. There are other types of oxidizing too, such as how the oxygen in our bodies is used to break down sugars. Oxidizing is a type of chemical reaction.