Your study will be in the preparation stage when you have just created your study and you are currently defining/specifying the different study settings such as your audience and tasks.
When you have defined your study settings as per above, and you publish your study, that means that your study is currently live and accepting interviews. If you want to change some study settings and un-publish your study the study status will change back to 'In preparation'
Ready for analysis
When you have received all of your required interviews, the study status will change to 'Ready for analysis' indicating that you can now begin analysing your data.
After you have completed your analysis, you can manually change the status of the study to 'Completed', in order to organise your and giving you a better overview of which studies are done, and which might still require analysis. You can complete a study by clicking the burger menu on the study.