Variable.

There is variable in program language,
it can substitute numerical value and character string.

Let's use.

10 ab=120
20 print ab

run
 120
OK

In this way,
120 is substituted for variable'ab',
and 'ab' is displayed as value of 120.
Anything is OK for variable name.

Let's use character string variable.

10 st$="tree"
20 print st$

run
tree
OK

Please attach '$' at the end of the variable name in string variable.