That is how feminine words pluralize if they don't end in ी.
Ends with ी: बिल्ली -> बिल्लियाँ
Doesn't end with ी: गाय -> गायें
Ends with ा: लड़का -> लड़के
Doesn't end with ा: बंदर -> बंदर (unchanged)
Hope that helps. Also be careful as there is also the 'oblique case' in which you get more complicated forms, but don't let those confuse you. The basic pluralizing scheme is pretty straightforward.