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. Percent is also used to indicate changes in numerical quantities.
For example, we have a bowl of fruit with three apples and one orange. The percentage of apples is 3 out of 4, or 3/4 = 75/100 = 75%.
A percentage is only one way of writing a ratio; one can also write it as a fraction or decimal. There are ways to convert fractions to percentages or decimals to percentages.
Percentages are useful in practice, because it allows one to compare things that are not out of the same number. For example, exam marks are often percentages, which can compare them even if there are more questions on one exam paper than the other.