WiFi is a wireless local network. Phones connect to WiFi using a Wireless Access Point. The SSID (Service Set IDentifier) names your access point (e.g. your home network).
WiFi uses frequencies in the 2.4GHz and 5GHz radio bands. To use both of these bands your phone needs to support 2.4GHz and 5GHz WiFi and you need a dual-band WiFi access point (router). Single band (2.4GHz) access points are still common. Using dual-band WiFi provides more bandwidth, can reduce problems caused by interference and can provide 2 dedicated networks (e.g. for general use and gaming).
The WiFi radio bands are split into channels. For example, in the United States, 11 channels are used in the 2.4GHz band (channel 1, 2… 11). WiFi connections can be disrupted or the speed slows when other nearby WiFi devices are using the same channel. Changing your access point channel (e.g. in your routers administration settings) or moving to a dual-band access point can help with this problem.
Each WiFi device is given an IP (Internet Protocol) address on the local network and this address is used to communicate with other devices on the network, typically via the Gateway (e.g. home router).