The first letter is the newest 802.11 standard the Router will support.
N means Wireless N (Totaling B, G, and N)
AC means Wireless AC (Totalling B, G, N, and AC)
AC wireless has the potential for much higher speeds on wireless, hence the next numbers. These are the speed (theoretical really) on wireless.
N300 is 300Mbps. If the router is single band, meaning it only runs on a single frequency (2.4 Ghz) it would have 300Mbps on that one frequency. If the router was dual band (a 2.4 Ghz, and a 5 Ghz network being broadcast at the same time), it would have 150 Mbps on each.
So, generally AC and a bigger number means a better router, but if you really don't know much about wireless networking, you may want to look at reviews of people...