A "user" is anyone who authors a pull request or merge request on a private repo with Codecov coverage, or accesses advanced features on Codecov.io for a private repo.
For example, anyone with access to your repos in Github/ Gitlab/ Bitbucket will be able to see Codecov data on the codehost if a user has authored a PR (an example from an open source project, scrapy, is here). Once you click through the Codecov comment you enter Codecov.io to see more robust metrics, historical trends, and coverage visualizations (example from the same PR here ).