Trash and restoring deleted items
Deleted leads are recoverable for 30 days before they are removed for good.
Deleting a lead does not remove it immediately. It goes to Trash, where you have a window to change your mind before it is gone for good.
Restoring
Open Settings → Trash to see soft-deleted leads. They are kept for 30 days; restore one and it returns to your pipeline exactly where it was.
Deleting for good
From the same page you can permanently delete an item before the 30 days are up. That cannot be undone, so it asks you to confirm. Anything still in Trash after 30 days is cleared automatically.