As you can see, the expression 4y9 is consisted of the numbers 4 and 9, and the letter y. In Math, we call this letter variable. We call it like that because the value of the variable is not fix. it varies. From this comes the name variable. To each value this variable takes, the expression gets a specific value.
For example:
If y=1, 4y9 = 4*1*9 = 36
If y =2, 4y9 = 4*2*9=72
etc...
Hope this Helps! :)