It's a Hardware Verification Language. As you might already know, hardware (computer chips) is designed using a Hardware Description Language (VHDL, Verilog) which is then synthesized into gates like NOR, NAND and sequential elements like Flip-Flops. So before you do synthesis, which is a tedious process, you would want to make sure that the functionality aspect of your HDL-constructed design looks good.
With System Verilog, you are able to create complex testbench structures and perform simulations to verify the design. You are essentially making sure that the code you have written with verilog functions well and implements features that you intend to. To know more about System Verilog follow link : Introduction to System Verilog