If you have ever wanted to use the wifi at a coffee shop or library, you have probably had to click through a screen to do it. This screen might have shown you the network’s Terms of Service and prompted you to click an “I agree” button. Depending on where you were, it might have asked you for information about yourself, like your email, social media accounts, room number (in a hotel), account number (in a library), or other identifying information. Sometimes you even have to watch a short video or ad before wifi access is granted.
These kinds of screens are called captive portals, and they interfere with wireless security without providing many user benefits.
Read more at EFF