In short
Ad blockers are browser extensions or network tools that strip ads from pages and, as a side effect, block marketing pixels and analytics scripts. That side effect is the part that matters for measurement: when a blocker removes a pixel, the conversion, page view, or event it would have reported simply never arrives, so client-side tracking undercounts by however much of your audience runs one. The share is not trivial, and it skews toward exactly the technical, privacy-aware users many businesses most want to measure. Ad blockers are therefore one of the main reasons browser-based measurement is unreliable, and one of the main arguments for first-party, server-side delivery, which sends events from a server you control rather than from a script a blocker can strip. You cannot stop users blocking ads, and there is no reason to try, but you can move measurement off the surface blockers target, so the events still reach the platform and the numbers reflect reality.
The measurement side effect
Most people install a blocker to remove ads, but blocklists also cover the pixels and tags that record conversions. The user still buys; the platform never hears about it; bidding models train on an incomplete picture.
What survives blocking
Server-side events do not pass through the browser, so blockers cannot strip them. Purchases, leads, and offline outcomes delivered through Conversions APIs reach platforms whether or not a blocker is present.
How much does an ad blocker remove from your data?
A site with a technical audience sees its pixel report noticeably fewer conversions than its back-end records, because many visitors block it. Sending the same events server-side recovers the measurement the blockers were removing.
Reference: Wikipedia, ad blocking