Without VPN: Your internet traffic goes directly from your device → Your ISP → Website. Your ISP can see everything you do.
With VPN: Your internet traffic goes: Your device → Encrypted tunnel → VPN Server → Website. Your ISP only sees encrypted data going to the VPN server, not the actual websites you visit.
Think of it like this: Without a VPN, it's like sending postcards through the mail - anyone can read them. With a VPN, it's like sending letters in locked, tamper-proof boxes - only the recipient can read them.