A bezel is a ring fitted over the caseband of the watch case. It is used to record additional data, such as the duration of a phenomenon for example. It can rotate in both directions.
The unidirectional bezel on the other hand, as its name implies, only turns one way. This technical constraint prevents any danger when measuring a diving time for example, since even if the bezel is knocked and moved it will simply indicate the diver has less air or decompression time rather than more.