Short answer: most changes you make on your live site will not be over-written by importing changes from your staging site, so it is safe to work on your live site (add new posts, etc.) while you are doing work on your staging site.
Long answer: File changes are handled differently from database changes. You can select which file changes to import, so you have control over which changes you import from staging. If you change a file on your live site (such as your theme’s stylesheet, or updating a plugin), and make changes to the same file on your staging site, and import that file change, the file from the staging site will overwrite the file on the live site.
Importing database changes works differently: instead of just copying the entire database from the staging site to the live site, WP Stagecoach records all of the actions (in the form of SQL commands) that happen on the database on the staging site and then does all of those actions to the live site when you import. So if you make a new blog post on your live site, and make a new blog post on your staging site, and then import your database changes, your live site will have both blog posts. Note: posts and pages that you create on your staging site might not have the same ID number when you import them to your live site.