Number sentences represent problems in mathematics.

For example, if you have $12$12 boys and $13$13 girls in a class, how many students are there all together in the class? There are $25$25 students in the class. This could be represented by the number sentence:

$12+13=25$12+13=25

When you see an equal sign in a number sentence it means that both sides have to balance, or have the same value. If both sides balance we say the number sentence is true.