The golemancer's bell is a control device for Thaumcraft golems that you have constructed. Without a Bell, most golems are useless: Those golems which don't use marks (Gather, Butcher, Guard, Fishing, Harvest) can be placed once, but then cannot be transferred to another location.
If you hit a golem with the Golemancers Bell, it will go back to its item form and it can be picked up again. Shift-clicking will cause it to drop all upgrades. Generally the core will be recoverable, but elemental upgrades may be lost, and accessories usually will be lost.
Right clicking on a golem however, will allow you to show the golem where it needs to work. For most golem cores, this consists of marking chests or other locations for the golem to work on. Note that the golem's placement ring is special, and can be changed only by picking up the golem and placing it elsewhere..
Trying to use the bell on a village's (vanilla) iron golem with the Bell will anger the golem.