While browsing on a secure WiFi connection like the ones at home or at school, your data is relatively safe; public WiFi, though, is a different story. On most public, unsecured WiFi, your data is open to anyone else connected, and you don’t necessarily know who set up the connection, so you don’t know how the connection is processing your data.
This is the WiFi you'll find in your home, at school, or in an office. These connections are generally secure, so you shouldn't worry all that much about your data being stolen while connected, but it's still good to keep your guard up just in case.
Public WiFi is the connection you'll likely find at a coffee shop, hotel lobby, or airport. These connections are generally not that secure and leave data open to hackers.
Ideally, Public WiFi should be avoided to sidestep all of these problems entirely, but that sometimes isn’t possible. If you have a deadline coming up, projects to finish, etc., public WiFi may be unavoidable. There are ways to stay safe while on public WiFi though.
“Public WiFi? More like don't steal my data please!”