The Graphics Processing Unit (GPU) is almost like it's own mini computer inside your computer, specializing in processing graphics and video. The GPU is closely related to the CPU of the computer. The GPU does many of the same tasks, however it focuses on making detailed graphics. It even has it's own RAM (VRAM, short for Video RAM) just for this purpose. Having a decent GPU is important for video gaming or for video and graphics rendering. Also, artificial intelligence (AI) companies have found a use for GPUs, as GPUs are used for generating AI pictures or videos.
Many modern computers and laptops have an integrated GPU embedded along with the CPU in order to enhance the versatility of the device. However, for those interested in gaming or video rendering, having a discrete (meaning, seperate and not integrated) GPU is more than likely a good decision, especially for higher-end builds. Luckily, GPUs are fairly standardized in the ways of naming, so generally, a higher number for a GPU generally means it's newer and more powerful, although this isn't always the case. It is important to do your own research on the intricacies and advantages of each GPU to understand which one is better for you. User Benchmark has a website that has an easy overview and comparison of GPUs to help you find one that's good for you.