Small metal studs fitted to ledges, benches and handrails that kill any grind on contact. They are the owner's unmistakable no — which means: find another spot.
Street skating lives off architecture that was never built for skateboards — which is exactly where its legal grey zone comes from. Skateboarding itself is not banned outright in most countries, but using a given place often is: on fenced private land it is trespass, and damaged benches and handrails are criminal damage. Skate stoppers are the built answer to that, and working around them or unbolting them usually counts as criminal damage in turn. The typical mistake is reading them as a challenge rather than as a statement. In practice: look for public space before company or private land, pack up calmly and leave if someone approaches instead of arguing, and take wax residue and litter with you. What usually gets missed — a spot doesn't survive your session, it survives the reputation you all leave there.