The delete action

The second action that is available in the "Advanced" action menu is the "Delete" action.

In the tree it's drawn as a red cross:

Hands that are accepted by this action are retroactively deleted.
It will be as if they never existed.

So what's the difference with a fold?

The difference with a Fold action is that a hand that is folded is still included in all the equity and EV calculations.
However, hands that are deleted are not considered in any calculations in any way.
For that reason, when you compute a tree with a Delete action in it, this action will always have a frequency of 0%.
The hands that went through it have essentially been erased from history.