If you want to delete many items from an app, use the bulk delete tool.
To delete many items at once, you need to view your items in the Table Layout as seen in the image below. You can either select all or select the specific items you want to delete, then click the small trash can in the upper right to delete the items.
Tip: Use the filter function to filter for a specific group of items, then delete all of the filtered items.
Before you delete you also need to type "delete these items" in the notification to confirm you actually want to delete all of them.
Remember that if you delete items from an app, the action is not reversible. Sometimes it might be better to save historical data and use filters and reports to save a view of your current items.