The time it takes for the first ad to be rendered. This is the interval from page load until the first ad impression is recorded.
This metric is not indicative of a specific issue. It should be used to help identify areas for improvement and track that improvement over time.
The goal here is to reduce the time it takes for the first ad to be rendered. Ensuring that other, more specific audits are passing should have a major impact on this metric. In particular, audits affecting tag load time and latency of first ad request (from tag load) are likely to affect this metric.