Using an on-camera TTL flash (aka “strobe”) properly is one of the easiest ways to take your photography to the next level. Once you have a DSLR and a lens or two (or three), probably the next piece of photographic equipment you’re going to want to buy is a flash.
Many DSLRs come with a built-in “pop-up” flash right on the top of the view finder. This flash can work OK in certain situations, but it can be limiting due to its low power and inflexibility. Buying a larger hot shoe mounted strobe will drastically increase the creativity you can get out of flash photography.