I open 1C -- Partner: 1D -- I bid 1S -- Partner bids 2NT - What 2NT means (System: Simple SAYC)
In this context 2NT is an invitation to game. Your partner probably has around 11-12 points, so he doesn't know if you have enough points between you to make 3NT (you haven't bid anything that shows more points than the bare minimum needed to open).

As your hand is pretty weak you should pass. If you had 14 or more points then you would bid 3NT.