In short, server-side ad injection means YouTube is adding advertisements to the video stream itself. Currently, the company delivers its ads to users as a separate video before the video you chose to watch.
This change would break its services, too, as adding ads to the video itself throws off the timestamps of the video. As ads vary in length and number, timestamp changes will be unpredictable, and tools like SponsorBlock won't work as they're currently designed.
When using certain ad blockers in some browsers, users saw a pop-up warning them to disable their ad blocker. If they continued to use their ad blocker, they may find that YouTube wouldn't load for them at all.
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