Exactly this. If you connect to your database and run a couple un-indexed queries to explore your data, any query which takes over 100ms will appear for the next 30 days in the "Performance Advisor" tab and it will offer you indexes to create targeted at that query. Just based on a query you did manually one time.

Just to note, this is referring to the features for a hosted databased on cloud.mongodb.com, and not something built into MongoDB the database.