In a calculation, it tells FileMaker to ignore it and all text to the right on that line. It enables you to insert a comment into your calculation.
In many script examples posted here in the forum, we use that notation as a way to add a comment to the reader for something that isn't supposed to be used verbatim in the script.
If [ Get (FoundCount ) = 0]
// do steps for what you want to happen when no records were found
From FMP Advanced help...
If you are using FileMaker Pro Advanced, you can disable script steps for testing. To disable a script step, select it, then click Disable. To enable a script step, select it, then click Enable. Disabled script steps are preceded by “//”.
Are you using Advanced? Can you? It looks like your script steps were disabled through a copy of Advanced. When using an FMPAdvanced copy, click on the script step, then click "Enable".