Somewhere between C++ concepts and Haskell typeclasses with a hint of Java interfaces.

But yes, their primary usage is to require that type parameters exhibit a certain set of properties.

For someone who knows C#, I would explain them as abstract extension methods that can be overriden for specific types.

They fill the same role as concepts - ie. bringing type checking to the call site when using parametrized types, rather than using a duck typed approach, which leads to the big template stack traces that you get in C++.

There are some technical details that make them different, if I recall correctly. But they're kinda similar. They're also close to Haskell's typeclasses.