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The term "worldspawn" is used by mappers (See also mapping.) to refer to the first entity of a map file. This entity holds all brushes (walls, floors etc.) of the 3D world that are not used by other entities (like automatic doors etc.).
Worldspawn brushes are "dead", they're just there, while the brushes of other entities fulfill a certain function depending on the entity. A door's brush, for example, moves around. There are also origin brushes (normal brushes with the texture "origin") which are used to define the anchor point of entities like e.g. a train or a rotating door.
A complete map usually has thousands of brushes in their worldspawn entity, and then there are lots (often hundreds) of entities which can also have brushes.
The worldspawn is not just the element that holds all world brushes. It can also hold definitions which alter the functionality of a map. See for example: G_disabledEquipment, G_disabledBuildables, G_disabledClasses. The amount of buildpoints available to the teams and the amount of kills necessary to reach stage 2 and stage 3 can also be defined there. All this can be read in the immediate help text available in the map editor NetRadiant.