A percent is part of something, split into a hundred pieces. For example, fifty percent of forty is twenty.
You'll hear a lot about percent and percentages in math. A percent can go from 0 percent to 100 percent. A percent is calculated by comparing a part of something compared to the whole. A baseball player's batting average is a percent. Athletes are known for talking about numbers that are impossible, like "I gave 110 percent!" You can do that in slang, but don't try it in math class.