Well in simple terms with a CG NAT is basically the main router to which your ONU is connected and you cannot change anything in the main router. So you cannot open any ports. P2P gaming is gone (especially on XBOX/PS/PC games which need open NAT type). You won't be able to host anything on your network (which is accessible from the public internet).
Can read more here: Carrier-grade NAT - Wikipedia
Can read more here: Carrier-grade NAT - Wikipedia