Spruce is a verb that means you pay special attention to your personal appearance, especially your grooming and clothing. If you want to spruce up for a wedding, you'll need to rent a tuxedo and do something with your hair.
Spruce can also mean that you're making something else look better. You might spruce up the living room before your girlfriend's visit by vacuuming, throwing away all the trash, and putting away the dishes you left sitting around. Spruce is typically used in combination with the word up: spruce up. Spruce's use as a verb possibly originated from "spruce leather," which was used to make a popular type of jacket back in the 1400s.