Mathematical expressions are a combination of numbers, variables (like x, y, or z), and operators (like +, −, ×, and ÷) that represent a particular value or quantity. For example, 2+3 is a simple mathematical expression. The + is an operator that tells us to add the two numbers together.

Imagine you have 10000 apples. Then, somehow you magically duplicate all of them, and now you have twice as many as before. Now, you have a total of 20000 apples. That's why 10000 × 2 equals 20000.

More Info: en.wikipedia.org