The Redo command reverses the effect of the Undo command. In effect, as you edit, Source Insight makes a list of the changes you've made to each file. The Undo command backs up through that list and the Redo command advances through the list.
You can also use the Redo All command to reverse all the Undo actions.
Source Insight keeps Undo/Redo history for each open file independently.