What Are Variables? — Definition & Examples

$a$ and $b$ can be any values, while there is only a single value $x$ that makes $x+1=2$ true.

$x$ can be any value, while there are only a few values of $a$ and $b$ that make $a+b=b+a$ true.

There is no real difference. Each of the variables $a,b,x$ can represent any value.