What does regexp.$1 mean in JavaScript?

REGEXP is a built-in object in JavaScript. is a regular expression.
Regexp.$1 is a property of RegExp that refers to the first sub-match (in parentheses) string that matches the regular expression.
And so on, regexp.$2,regexp.$3, ... Regexp.$99 can have a total of 99 matches

As an example:

var // Regular expression matches birth date (simple match)      R.exec (' 1985-10-15 '); s1=regexp.$1; S2=regexp.$2; S3=regexp.$3; alert (s1 + "" +s2+ "" +s3)// result is 1985

What does regexp.$1 mean in JavaScript?