g is for global search. Meaning it’ll match all occurrences. You’ll usually also see i which means ignore case.
Reference: global – JavaScript | MDN
The “g” flag indicates that the regular expression should be tested against all possible matches in a string.
Without the g flag, it’ll only test for the first.
Additionally, make sure to check cchamberlain’s answer below for details on how it sets the lastIndex property, which can cause unexpected side effects when re-using a regex against a series of values.