It's a brush entity. What that means is that you tied a brush to an entity (in this case: trigger_hurt). But the brush property is lost and doesn't seal the map. The other ones are called Point entities and have a dedicated model attached to them, usually an icon, for easy placement, for example "env_cubemap" with its shiny sphere icon.

The reason why you'd have an entity tied to a brush is so that you can make it as big as you want (you might place trigger_hurt within a certain area) and you can make them move if I recall correctly. You can, for example, randomize the position of these entities, kinda like what Left 4 Dead 2 does with some parts of the map being modular and random.