Introduced quite a while back for polylines and a while back for hatch, these allow you to add, edit and stretch existing lines by editing the blue square grips.
However sometimes on polylines the edit menu for removing does not appear. This is usually due to two overlapping grips being present in the same space.
To solve this problem, move the grip between the two points that don’t work to reveal the hidden overlapping grips. Now you should be able edit the grips again.