Many times we run into situations when we need to have different VLANs on a single Ethernet port. So you have a wire (or port on a router/switch) on which packets travel for two or more different networks but are not able to talk to each other. To make them talk to each other, you have to enable routing on the router or on the Layer 3 (L3) switch. In this situation you have this router acting as a “router-on-a-stick” when it routes packets from/to different VLANs connected on a single physical port.
Hope this clarifies.