WiFi SSID is the name that identifies a WiFi network. This is unique to each network. In Regular.li, WiFi SSID ensures that Members' Checkins/Checkouts are allowed only when their handheld devices are connected to a particular WiFi signal.
A Location can be mapped to multiple WiFi signals. Members mapped to this Location can then be allowed to punch their Attendance from this Location only if their handheld devices (mobile phones/tablets) are using any one of the signals. In order to enforce this restriction, WiFi SSID should be enabled in Member's Security Settings as well as in the Location's Settings.
WiFi SSID comes in handy in places with low GPS availability like shopping malls or deep indoor office spaces.
You can watch a tutorial on Checking In and Out by clicking here.