Using the PUT function
One form of a legal expression in an assignment statement is a function. The assignment statement includes what is called a "function call". That is, in
GRADE = Put (Score, Score.);
the expression tells SAS to call the PUT function and pass it the arguments: the variable Score and format Score. .
SAS functions (data step expressions)

Commas (,) delimit multiple arguments.
The arguments may be any valid SAS expression.
The values of the arguments are passed to the function. The function uses the arguments and returns a single value.
For more information, see SL:R, Chapter 3 Components of the SAS Language, SAS Functions and SL:R, Chapter 11, SAS Functions.
There are many builtin functions in SAS (i.e., SQRT(X)or MAX(T1,T2)).