Variation is alternative continuation for game.
New variation (alternative continuation) for move is added using command Insert -> New Variation for This.
Variation can be seen in game tree in bottom right part of screen.
Several variations can exists for single move. In order to add second variation, go to move for which variation is desired and use command Insert -> New Variation for This. Second variation is marked with <B> in game tree.
Variations can be shown before or after actual move. This selection can be done in View -> Show Move Before Variation. This selection corresponds to SGF file modes "variation as sibling" and "variations as children".
Variations can be deleted, when current move is in variation.
If you accidentally delete variation, you can use Edit->Undo to restore it immediately.