How placement exclusion works
Display and video campaigns serve across an enormous pool of websites, apps and YouTube channels. A placement exclusion removes named items from that pool. Add a site, an app or a channel to the list and the campaign stops bidding on it, regardless of how well it matches your audience or your keywords.
Exclusions can be set on a single campaign, or held in a shared list applied across the whole account. The shared version is the one that matters, because it means an exclusion you learn about in one campaign protects every campaign you run afterwards, including ones that do not exist yet.
You are not limited to naming individual placements either. You can exclude broad classes of inventory, mobile apps as a group being the most useful example, along with categories of content you would rather not sit beside. That distinction between naming items and removing categories is the difference between a list you maintain forever and a list that mostly maintains itself.
Why placement exclusion matters
Two problems make it necessary. The first is accidental clicks. Games and utility apps place ads where fingers already are, so they generate clicks from people who never intended to visit anything, and those clicks cost the same as real ones. The second is content adjacency: a site whose subject matter you would not want your brand associated with, or a channel that adds nothing but volume.
There is a third, quieter reason. Sites built purely to carry advertising exist in large numbers, and they absorb display budget without ever sending a buyer. Excluding them is often the single largest improvement available to a display campaign, and it costs nothing but attention.
Where placement exclusion goes wrong
The usual failure is treating it as an endless game of removing one name at a time. New inventory appears faster than any person can block it, so a list built name by name will always be behind. Excluding categories and whole classes of inventory does far more work.
The second failure is excluding by feeling. A site with an unappealing name may be converting well; a respectable one may be producing nothing but accidental taps. Read the report before deciding. The third is applying exclusions at ad group level, where they protect one ad group and nothing else, when the same list at account level would have covered everything.
How to act on it
Build one shared exclusion list at account level and treat it as a permanent asset. Everything you learn in any campaign goes into it, and it applies from that moment on across the display network and video inventory.
Review the placement report on a set cadence rather than when something looks wrong, because wasted display spend is quiet by nature and rarely announces itself. Look for placements with heavy spend and no conversions, and for click rates that are far out of line with everything else, which usually indicates accidental taps rather than interest. Then pair the list with deliberate placement targeting so you are steering towards good inventory as well as away from bad.