The process of letting the glass cool very slowly so that internal stresses are removed and the glass is tougher than it was. What annealing is to us as glassblowers- This can mean a lot of things to us in the glass blowing world, yet still means the same thing. We anneal the glass in order to remove the stress. The stress is built up in the glass as some parts are hotter than others and the molecules move differently depending on if they were hotter or cooler. What we are doing when we anneal is to bring all the molecules up to the same temperature so they all dance the same. This is not actually annealing though. The annealing starts as we cool the glass. The slower the better. This allows all the molecules to cool at the same rate. This way the molecules arrange in a more uniform way. The thicker the glass the slower the glass much cool to become fully annealed. Borosilicate glass anneals at 1050 F. Soft glass (Moretti) anneals at 960 F.
You can learn more by going to glass terms and techniques page and looking for the topic.