The efficiency of a resource can be used for two purposes. First you can use it as a crude way to model a team. A team of 5 people should have an efficiency of 5.0. Keep in mind that you cannot track the members of the team individually if you use this feature. They always act as a group.
The other use is to model performance variations between your resources. Again, this is a fairly crude mechanism and should be used with care. A resource that isn't every good at some task might be pretty good at another. This can't be taken into account as the resource efficiency can only set globally for all tasks.
All resources that do not contribute effort to the task, should have an efficiency of 0.0. A typical example would be a conference room. It's necessary for a meeting, but it does not contribute any work.