They're biomechanical weapons. We had to come up with what would be the ultimate version that could actually, in principle, stop the X-Men. We started with this idea that they were almost made up of magnetic plates slapped over one another, imagining that the plates could contract or grow, so the Sentinel can be skinny to get through a small space or the plates can open up to become a bigger shape. So they have become virtually unstoppable.
Check out a 1973 Sentinel followed by the future version: