here's a brief explanation:
myproject - this is the staging sandbox where approved content ends up
myproject–admin - this is the user sandbox for the admin user which is where admin will author content
any sandbox name ending in –preview (myproject–preview, myproject–admin–preview) is a layer used for work in progress, meaning, that when in the process of creating content using forms, you can preview all renditions generated in context within the preview layer before committing those changes into the author layer (by clicking finish within the wizard).