With WiFi, you can have multiple networks on the same channel. They are separated by their SSID (name) and BSSID (MAC address). It’s like VLAN tagging, only mandatory.
These networks can be served by a single radio. A radio can operate on only one channel (concurrently). Of course, all devices on the same channel (or with overlapping channels) share bandwidth, so you won’t get, say, N×300 MBit/s. Popular consumer-grade WiFi chipsets offer support for anything from 2 concurrent networks to at least 8 networks.
All networks can have different levels of encryption/access control set. However, they share hardware settings like transmit power and whatnot.
Inside the router/access point, these networks appear as separate network interfaces and can be linked to, say, a tunnel to your ISP.