Captive Portal is a web page which is displayed to users when they are connected to a public-access network, based on which access is restricted to the network resources.
Captive portals are primarily used in open wireless networks where the users are shown a welcome message informing them of the conditions of access (allowed ports, liability, etc.).
Captive portals are typically used by business centers, airports, hotel lobbies, coffee shops, and other venues that offer free Wi-Fi hot spots for Internet users.
Captive portals are commonly used to present a landing or log-in page which may require authentication, payment, acceptance of EULA/accepted use policies, or other valid credentials that both the host and user agree to adhere by.
Captive portals are used for a broad range of mobile and pedestrian broadband services - including cable and commercially provided Wi-Fi and home hotspots.
For more details, please refer the wiki page Captive portal.