Similar to sandpaper, grip tape, or “grip” as it’s commonly referred to, is applied to the top of your deck for traction. Grip gives you the friction you need to perform tricks such as ollies and kickflips. Not all grip tape is the same however. Each brand's products perform differently, so you may want to experiment with different brands to get a feel for what you prefer.
Without good grip tape, skaters have a tough time staying on top of their board. The main complaints about poor grip tape tend to be that it was never very grippy or lost its grip really quickly. Once again, we recommend trusting in name brands with years of experience supporting skateboarders. Unfortunately, if you are buying a pre-made complete skateboard, you probably won't have access to the brand name of the grip.