A VPN (Virtual Private Network) creates an encrypted tunnel between two endpoints over an untrusted network. Think of it as a secure pipe running through the public internet. Your original packets get wrapped inside new headers and encrypted before transmission. The receiving end decrypts and unwraps them.
Without a VPN, your traffic travels in plaintext across networks you don't control. Anyone with access to intermediate routers could read or modify your data. A VPN ensures confidentiality and integrity end-to-end.