A captive portal is a webpage that is presented to the user before they are fully authenticated to a network. Captive portals can require a user to authenticate using a username and password, accept terms and conditions of use, or provide payment for use of a computer network before the user is allowed to use that computer network. KenyonWifi, KenyonDevices, and KenyonGuests require a user to enter a username and password to authenticate to the network, while KenyonPublic only requires a user to accept terms and conditions of use before allowing access to the network.
On macOS, the captive portal is automatically loaded when you try to connect to Kenyon's network, which requires a captive portal. This shows up as a small window with a login page inside of it. On Windows, you are redirected to the captive portal if you try to visit a webpage in a web browser without having authenticated first.
An example of a captive portal.