Definition
An Internet Service Provider sells access to the internet, typically over cable, fibre, DSL, satellite, or cellular. ISPs see every connection their customers make: every domain looked up, every server connected to, every byte exchanged. Even when individual sessions are encrypted with HTTPS, the metadata (which sites, when, for how long, how much data) is fully visible to the ISP and is logged for varying periods depending on jurisdiction.
Why VPNs Concern ISPs
When you use a VPN, your ISP sees only encrypted traffic to a single VPN server IP. They cannot see which sites you visit, what services you use, or what content you view. Some ISPs respond to VPN usage with throttling, others with marketing-oriented monetization of the small subset of customers not using a VPN. In jurisdictions where ISPs are required to retain or share data with authorities, a VPN substantially blunts their useful contribution to that surveillance.
Trust Shift
Critics rightly point out that using a VPN does not eliminate the trust requirement — it simply moves it from your ISP to your VPN provider. The argument for the shift is that you can choose a VPN provider in a privacy-friendly jurisdiction with a no-logs policy, while you usually cannot choose your ISP. For users in markets with regulated, log-friendly ISPs, the trust shift is a clear improvement; for users with a trustworthy ISP, the gain is mostly metadata-hiding rather than confidentiality.