A venturi is a restriction in the carburetor air horn. Air is forced through the venturi and when it goes through the restriction (B), the air speeds up. This creates a vacuum which will be strongest in the throat of the venturi, but will exist for a short distance after the venturi (C). This is not the vacuum source for things like the vacuum advance, but does play a crucial part in drawing fuel from the carburetor float bowl.
2 Jet Venturi Example
Edelbrock 4 Barrel Venturis