Wi-Fi evil twins

An evil twin is a wireless network pretending to be one your device already trusts, using the same name.

Devices are helpful and reconnect to familiar networks automatically, so this frequently needs no action from you at all. Once connected, the person running it decides what you can reach and what you are shown, which is the modern public Wi-Fi risk: not reading your traffic, but steering it. A network prompting you to sign in again, or to install something, deserves far more suspicion than it usually gets.