A VPN, virtual private network, gets talked about like it's a magic cloak of invisibility online. It isn't. It's a genuinely useful tool for a specific job, and understanding that job means you'll actually use it when it matters, rather than everywhere or nowhere.
What it actually does
A VPN encrypts the connection between your device and the internet, so anyone else on the same network, say, public Wi-Fi at a cafe or an airport, can't easily see what you're sending. That's it. That's the job.
A VPN protects the trip your data takes. It doesn't protect you once your data arrives.
What it doesn't do
A VPN doesn't stop a scam text from being convincing. It doesn't stop you from typing your password into a fake website. It doesn't make a weak password strong. It's protecting the journey your information takes across the network, not the decisions you make once you're logged in somewhere.
When it's actually worth using
Public Wi-Fi is the clearest case, a cafe, an airport, a hotel, anywhere you don't control the network. At home, on your own internet connection, a VPN matters much less. It's a specific tool for a specific situation, not something that needs to run every minute of every day.
Related reading
See how this connects to public Wi-Fi risk specifically in the post on cafe Wi-Fi.