Chart Version : When bumping the chart version follow semantic versioning https://semver.org/
App Version : When bumping the app version you will also need to bump the chart version too. Again, follow semantic versioning when bumping the chart.
To determine if you need to bump the MAJOR, MINOR or PATCH versions you will need to review the changes between the previous app version and the new app version and / or ask for a maintainer to advise.