Health and safety in the garden.

I went for a stroll round the garden to add up the hours of work I need to do before it becomes classed as overgrown. There's a lot of building work going on out there at the moment and as I went to walk in the door I noticed this rather strange 'bit' hanging. Bloomin' good job i'm not 7ft tall because it would have speared me through the eye.
So I asked hubby to explain it to me..... apparently.....
It is a catenary (?????) and it's hanging there because he removed the bit it was attached to i.e the front of the garage (do we not need the front of the garage then?).
Still means nothing to me but I will be putting up a sign for tall people to wear a hard hat and eye protection when entering the garden.