Broken rocker bolt.

Drill it out and remove with an easy out . You can get them at sears, harbor freight, or anywhere that sells tools. You will need to make sure shavings do not get down in the engine if you do it while the head is installed on the block. If part of the bolt is sticking up, you may be able to cut a slot in the broken bolt and remove with a screw driver. Take one of the other bolts to the hardware store and get a replacement. I would use at least a grade 8 bolt.