To braid is to lace or weave together. Hair that has been done this way is in braids.
Have you ever seen someone with their hair bound together in thick, rope-like arrangements? That hairdo is called braids. Women often braid each other's hair or get it braided by a hairdresser. The longer your hair, the longer your potential braids. To braid hair, you have to weave the strands of hair together: this is also called plaiting, lacing, and interlacing. Other materials can be braided, but the most popular thing to braid is definitely hair.