Hi krass, a "closed" solid means an object that is made up of set of surfaces joined to one another so that every edge of every surface is joined to another surface. An object created like that then forms a closed watertight skin that defines a volume.

If you have some surface edges that only belong to one surface (these are called "naked" edges) then that means your object does not form a closed volume and it's an open surface instead.

- Michael