Public Wi-Fi networks — in parks, cafes, airports, — are often either completely open (not password-protected) or not very secure. Connecting to these networks means that you share internet access with everyone around you. A stranger sitting at the next table might steal your data by using a simple, widely-available application
Online criminals also often use a different strategy — creating an open network of their own, with an easily-understood name (e.g. FREE_WiFi) and funneling everyone who connects to it through their own devices.