Wi-Fi security · Plain-English answer
Wi-Fi sniffing is capturing the wireless packets a network broadcasts, using software any laptop can run. On open networks, unencrypted traffic can be read directly. HTTPS has made sniffing far less rewarding, since page contents are encrypted, but domain names, device identifiers and unencrypted app traffic can still leak.
Wi-Fi is radio, and radio is broadcast. Every packet a network sends travels through the air past everyone in range, and freely available tools can put a laptop's wireless card into a mode that records all of it. On an open network with no password, those packets arrive unencrypted at the radio layer, so whatever the applications did not encrypt is readable.
Password protected networks encrypt the radio link, which shuts out outsiders, though anyone legitimately on the network still shares the medium and, on poorly configured networks, can observe more than they should.
HTTPS has hollowed out the classic attack, since passwords and page contents captured from the air are now ciphertext. What still leaks is the metadata: DNS lookups showing which sites you use, device names and identifiers, and traffic from the shrinking set of apps and gadgets that skip encryption, with cheap smart home devices among the usual offenders.
A VPN counters sniffing directly by encrypting everything at the device before it reaches the radio, leaving a captured stream with nothing legible in it. On open networks that is the difference between a partial diary and white noise.
ROOT VPN is free to start with unlimited data, no card needed, and no activity logs.
Get ROOT VPN, free