ok as for your set up i would probably suggest you change your crushed coral (CC) over to sand. this is because in the long run this will produce excess nitrates in the water from excess waste collecting on it. i would also suggest a Deep Sand Bed of 4-6 inches deep as a filter instead of the 2 filters. along with the DSB
i would try to get about 1-2 lbs per gallon of live rock to also help with filtration. This is a more natural filtration system that alot of people are using now days.
with that said a sump is just an area underneath the main display tank that adds water volume to your system. what you do is simply plumb your tank so the water cycles underneath your tank into a seperate tank so you can place unsitely equipment such as the skimmers and heaters for you tank. this area also alows you to top your water off in the sump instead of the main tank. because the water from the main tank overflows into the sump your display tank never appears to loose any water. some people also add refugiums along with there sump as a place to grow pods
a sump is not manditory but a personal preferance. IMO
i have had both types of systems with sumps and without and i prefer a sump but it is not a necessaty.