Definition of Variable
The properties of numbers are often written with the help of letters.
For example,we can write the commutative property of addition as $$${a}+{b}={b}+{a}$$$.
In this equality we can plug any numbers instead of $$$a$$$ and $$$b$$$: $$${32}+{4573}={4573}+{32}$$$, $$${350}+{3}={3}+{350}$$$ etc.
Number, that is plugged instead of a letter is called its value.
Letters, that are used in algebra, are called variables; they are called so, because we can change value of a letter.
For example, we can set $$${a}={3}$$$, $$${b}={5}$$$ or $$${a}={9}$$$, $$${b}={4}$$$ in equality $$${a}+{b}={b}+{a}$$$.
Conclusion. Letters are used to write variables. If it is not a variable (for example, number), then it is called a constant. Value of a constant can't be changed.
Usually, letters $$${x}$$$ and $$${y}$$$ are used for writing variables.
$$${\color{red}{{{2}}}}{\color{blue}{{\times}}}{\color{green}{{{x}}}}{\color{blue}{{+}}}{\color{red}{{{3}}}}{\color{blue}{{-}}}{\color{green}{{{x}}}}{\color{blue}{{\times}}}{\color{green}{{{y}}}}{\color{blue}{{+}}}{{\color{green}{{{x}}}}}^{{\color{red}{{{2}}}}}{\color{blue}{{+}}}{{\left({\color{red}{{{3}}}}{\color{blue}{{-}}}{\color{green}{{{x}}}}\right)}}^{{{\color{green}{{{y}}}}}}$$$
Constants are red, variables are green, operations are blue.