Bristly things are spiky or barbed, like a porcupine. When you get an old-fashioned shoe shine, your shoes will be cleaned with a bristly brush.
You can describe a certain type of hair as bristly — if it sticks up in tufts and spikes like a hedgehog's quills. A man's beard stubble is bristly too, and so are spiky bushes and scruffy dogs. Figuratively, if you're bristly, you're cranky and easily irritated: "He spoke carefully, trying not to upset his bristly grandfather."