Since I had to go looking for this recently, I thought I’d re-post it here. Comes directly from here SPSS for Dummies Cheat Sheet.
“You can insert descriptive text, called a comment, into your program. This text doesn't do anything except help make things clear when you read (or somebody else reads) your code. You start a comment the same way you start any other command: on its own line by using the keyword COMMENT or an asterisk or an asterisk-slash. The comment is terminated by a period. For example:”
COMMENT This is a comment and will not be executed.
* This is a comment and will continue to be
a comment until the terminating period.
/* This is a comment and will continue to be
a comment until the terminating asterisk-slash */