Most takeoff edits (even deleting takeoff or most Annotation) are "undoable", meaning if you do something to a takeoff object that you did not intend, you click the button to return to previous state, at least until you switch to a different Page.
However, deleting a Takeoff Item is permanent and immediate and affects the entire Bid. All takeoff drawn with the selected Takeoff Item(s), on every Page in this Bid, is permanently deleted - there is no 'Undo' available. Any time you delete a Takeoff Item, you are required to confirm the action - please consider what you are doing before you click "Delete Takeoff Item(s)" in that confirmation prompt. You (or a coworker who may have been using that Takeoff Item without you knowing) can lose days or weeks of work if you click the button without considering the consequences.
As a rule of thumb, when in doubt, DON'T!
Deleting Named Views, Punch Items, and Typical Group Frames are permanent actions also - you are warned about the deletion, do not confirm the action without considering the consequences - you can lose a lot of work if you click "Delete All".