Ventilation means the supply and extraction of air. This is necessary to guarantee fresh air both for inside your growing room or growtent and outside as well.
The plants need fresh oxygen and carbon dioxide for growing. Also the consumed air has to be extracted. Another factor is the regulation of the temperatur. Especially small growing spaces can overheat easily affecting the plants or even the technical components.
Typically it suffices to install an exhaustion system to extract the consumed air. Fresh air is automatically drawn into the growing room by doing so. This is optimal for smaller spaces (one or two lamps). Furthermore a moderate vacuum is created which means just a minimum of air leaving the growing space by other ways -> less smell.
Only bigger spaces or if you wish to completely airtighten the grow-room an additional air supply installation is needed.