Question

What is algebra?

Answer 1

What is the missing number?

Well, in algebra we don't use blank boxes, we use a letter (usually an x or y, but any letter is fine). So we write:
x -2 =4
It is really that simple. The letter (in this case an x) just means "we don't know this yet", and is often called the unknown or the variable.
And when we solve it we write:
x =6
Algebra uses letters (like x or y) or other symbols in place of unknown values.