Server-side tracking sends conversion and event data to ad platforms from your own server rather than the user's browser. It is more resilient to ad blockers, cookie limits, and browser restrictions than client-side tags alone.
Why move tracking to the server
Browser-based tags are increasingly blocked, capped, or stripped of cookies, so client-side measurement leaks. Sending events server-to-server, through Meta's Conversions API or a server-side tag container for Google, recovers events the browser would have dropped and gives you more control over the data you share.
Not a replacement, a partner
The common pattern is to run client-side and server-side together with deduplication, so each event is counted once. Server-side is what keeps a pixel-driven audience and Smart Bidding well fed as third-party cookies fade, and it works alongside Consent Mode, not instead of it.