Normally, the process of editing the latest version of an object and unlocking it so that it becomes the new [current] revision is exactly what you want. There may be situations however that you do not want to proceed with the latest version of an object. This may happen for example if the latest version turns out to have a bug that is easier solved in an earlier revision. Or when the [current] revision contains some prototype code that should not be developed further.
If that happens, you should make an earlier revision the [current] revision. You can do this by selecting this earlier revision, right-clicking, and selecting Make current. If you then edit the object, the new development revision will be based on the earlier revision that is now marked as [current].
The result of this is that the old [current] revision has become the end of a dead branch of development. Unless you explicitly make it [current] again, it will not be chosen anymore for editing. The new [current] revision on the other hand, has become the start of a new live branch of development. This can be regarded as a form of 'undo', or if you like, 'backtracking'.
This means that at any point in time, there can be multiple dead branches, but only one live branch.