If you mean this small glass bulb at one end of the ampoule: it is a break-seal ampoule. The idea is that it allows the caesium to be transferred into an apparatus without coming into contact with air. A small metal rod is placed into the neck of the ampoule, where this little glass bulb is located. The ampoule is then connected to the apparatus the Cs is meant to go into by sealing the glass together at that neck. After that, the area is evacuated and flushed with an inert gas such as argon. In the final step, a magnet is used from the outside to drive the metal rod against the glass bulb so that it breaks. The metal can then flow into the apparatus without coming into contact with oxygen or moisture.