Search conversion rate is the conversion rate of sessions that included at least one search, usually reported alongside the rate for sessions that did not.
The comparison is the point. Search sessions routinely convert at a multiple of non-search sessions, because searching is an act of declared intent. That ratio is the argument for investing in search, and it is measurable in most analytics tools without any new instrumentation.
It should be paired with search usage rate — the share of visitors who search at all. A site where few visitors search does not have low demand for search; it usually has an unfindable search bar.