Cold Start Session Percent is the percent of sessions that have a cold start. The rate is a baseline and deviations from the baseline can be interesting. For example, a sudden increase in cold starts could mean that there are more instances where the app is getting closed in the background and the user has to start from scratch. 30-40% is a pretty standard rate.
This metric is included on the Version Comparison dashboard by default or can be added to a custom dashboard. To learn more about how to create custom dashboards, reference this guide.