a form of barbed wire (or razor wire) that is delivered in coils. The concertina wire (that takes its name from the accordion-like musical instrument called concertina) solves the main problem with barbed wire, that's to say slowness of deployment.
Picture a Slinky toy, in its relaxed state: that's to say a cylindrical helix. Put it sidewise on a table, and pull on its ends. There, you have just deployed a roll of concertina wire!
Concertina wire comes in rolls that can be up to one meter high; to make stronger barriers they can installed in groups, with a cross section that looks more or less like this:
O
O O
O O O
(each O is a roll). Some types come in "clipped" form, that's to say, individual loops of wire are attached to each other in certain points, in order to strengthen the barrier and make it more difficult to cross.