What does WLAN mean in some iPhone it is used instead of WiFi

WLAN stands for Wireless Local Area Network. In general it means the same thing as Wifi when used instead of Wifi.

"Connect to a Wifi network", or a Connect to a Wireless local area network" are generally the same thing.

The term that shows up will depend on the language selected for the device. Some localizations of the english language will use WLAN instead of Wifi.

Where are you seeing "WLAN" and on what iPhones vs. the other iPhones?