Pressing the key "L" while over an object displays the word "Locked".
This locks the object under the cursor, preventing it from being deleted.
Caution: Do not press the Delete key while the cursor is over a locked object ,as an object(s) directly behind the locked object may be deleted.
But a bigger concern is the Lock function does not prevent imported meshes from being deleted.
The tree is locked.
Cursor over the tree and delete pressed