From QB64.org wiki
REM or an apostrophe is used for programmer remarks, comments or to stop the execution of program code.
- REM program comment or ignore code
- Comments cannot be read by Qbasic correctly and may cause syntax and other errors without REM!
- Instead of REM you can use the ' symbol which can be put anywhere.
- Code can also be commented out for program testing purposes.
- Qbasic Metacommands such as $DYNAMIC and $INCLUDE require the use of REM or the apostrophe.
Example: Avoiding an END IF error.