Graphics Processing Units are a special chips designed to handle video processing in computers or handheld devices which involves complex maths calculations
In simple words one can say that the CPU is the one that makes decisions based on a certain logic and the GPU is the one that does the repetitive hard work (mathematical calculations involved in video processing).
A GPU has several hundred cores to perform repetitive tasks in parallel i.e. the same instructions are executed repeatedly with different sets of data.
In the domain of IT security GPUs are important because cryptographic operations are mathematical functions that can be performed very well using parallelism. GPUs often find their use in cracking passwords which involves calculation of cryptographic hashes