Percent means out of one hundred. It is often shown with the symbol "%". It is used even if there are not a hundred items. The number is then scaled so it can be compared to one hundred.
For example, I have a bowl of fruit with three apples and one orange. The percentage of apples is 3 out of 4 = 3/4 = 75/100 = 75%.
A percentage is one way of writing a ratio; you can also write it as a fraction or decimal. There are ways to convert fractions to percentages or decimals to percentages.
Percentages are useful because people can compare things that are not out of the same number. For example, exam marks are often percentages, so people can compare them even if there are more questions on one exam paper than the other.