When you want the regular expression to apply to multiple lines separated by newline characters, this property should be set to true. When it is false the pattern is applied only to a single line.
Since this is a class property (a static property), it belongs to the global built-in RegExp object. This means it is shared by all RegExp object instances and therefore is very transient and will be overwritten as soon as the next regular expression is evaluated. If you want to preserve the value, you will need to copy it immediately before your regular expression has been evaluated and before you call another.