What are explanations in Google Ads?

Google groups them by cause: bidding, budget, conversion, conversion value, rejection rate, and asset, asset group and ad group explanations, plus others that include targeting changes, auction competition and search interest, change history and day of week effects. It notes that some of these explanations are not applicable to all campaign types.

By default, Google says you'll get explanations that compare the date range that you selected with the previous period of the same length. You hover over values in your data table that are coloured in blue with a dotted line, or over the performance graph, and click View explanations. They can also be viewed in the bid strategy report. For custom date ranges, Google says both periods must be of equal length and contiguous, or explanations won't show.

Google lists common reasons an explanation may not be available: the performance metric isn't aligned to your campaign goals, the change isn't significant enough to trigger an explanation, or the date range includes today or goes further back than the last 90 days.

Our reading, not Google's: an explanation is a cause to check, not a verdict, and a missing one is not proof that nothing changed. Google's list includes a drop in conversion tag fires, but an explanation can say the tag fired less, not why, and nothing on the list covers what happened to the leads afterwards, so check the site and the leads too.