>what means: case sensitive ?

Upper-case (capital letters) and lower case 9small) letters are not the same. IOW:

"ABCDE" <> "abcde".

The most common place for this in VFP coding is in declaring the name of an DLL entrypoint in the DECLARE...DLL statement; GetUserName is not the same as GETUSERNAME in the DECLARE statement, but they are equivalent elsewhere once the function has been declared.