Tan is a light brown color you might also call "beige." If you want to brighten up your bedroom, you can paint your boring tan walls with green, yellow, and hot pink stripes.
Some people who spend a lot of time outside in the sun end up with a tan, a darker skin tone than they usually have. The verb tan means to deliberately soak up the sun in the hopes of getting a tan. You can also use tan as a verb to describe the process of making leather from animal skins: "Then I'll tan the deer hide by soaking it in this solution."