Delve is a powerful tool to see and discover information that is likely to be interesting to you. You only see content that you have access to. The powerful search behind Delve uses the Microsoft Graph.
You can access Delve from the Waffle inside your Office 365, or by going directly to the URL https://delve.office.com and signing in with your 365 account.
Now you or someone else will only see a document if they have access to it. However, there might be a reason that you don’t want it to show up in Delve at all. To prevent a document showing up in delve you can create a HideFromDelve column of the type Yes/No in the library the document is in. The column creates a new crawled property within search schema called ows_HideFromDelve. This is automatically mapped to the HideFromDelve managed property. Set the column value to Yes for the document you do not wish to see in Delve.
Set the default value to No otherwise every document you create within the library will not show up in Delve (Unless this is the desire outcome).