Wireless AC is the new standard in wireless (or wi-fi as popularly known) that has appeared in last few years and is growing more popular. It has higher speed capabilities.
Previous versions were A, B, G and N. G was backward compatible with B and N was backward compatible with G and B. A ran on 5 ghz and B, G and N use 2.4 gnz which is just the wave length involved like the number to your radio stations. N was able to do 5 also but not well.
AC will work at 5ghz or 2.4ghz although it has more capabilities when running at 5 than at 2.4. AC can be compatible with A, B, G, and N although to do that it is sending out two signals.
Basically G was limited to speed of 54mbps and N was limited to about 120mbps. (there were boosters that did get faster if all same brand equipment) . mbps is megabits per second and is how many million bits are sent in one second in ideal conditions. All of the types are part of the IEEE 802.11 standard which covers wireless.
AC will allow transmission speeds up to 1gbps and are expected to go faster when using multiple devices. A single device connection is basically limited to 500mbps right now. Initially they are not those speeds. It is using a multiple channel approach so the wireless switch can actually talk with several devices at one time instead of being one at a time as was in previous versions. However the one at a time worked fine as it keeps flipping thru devices and you do not realize it cant chew gum and walk at the same time. Using the 5ghz band will be much faster than the 2.4 ghz but non-AC and A devices can not use 5. The AC wireless switch can be set up to use 5 and 2.4 ghz which is how I have my AC boxes set up. However with the increased signal in 5 the signal will not travel as far. However the signal strength is stronger before drop off so it will cover its area better.
The price on AC switches was initially much higher, but has come down a lot in the last 6 to 12 months.
Another advantage is there are more channels available on AC than G and N so you can use your switch without crossing traffic of your neighbors by finding a different channel and improve quality and speed.