Ad blockers are softwares specifically developed to remove or prevent ads from being shown on a website. When online there are platforms that communicate back and forth (Data management platforms to demand-side platforms, ad exchanges, to supply-side platforms. This is in addition to Ad networks and Ad servers). 

Each time a creative is presented, the platforms must communicate among themselves 

• Browser to ad server via the ad request 

• SSP returns an ad markup that loads the creative from the winning DSP’s ad server

• The browser loads the creative from the CDN

And so on… 

This communication is marked using tags so that the different players can ‘pass control’ each other. And, at any point it detects a domain to block, it jumps in to do its job. Ad blockers may ‘hide’ a creative even after it has loaded onto a page, but usually works by blocking elements with labels that give clues on where they are being used for advertising purposes.