Many organizations use Windows network file shares or Network-Attached Storage (NAS) solutions that rely on a combination of share and NTFS permissions to manage access. SharePoint sites, lists, and libraries similarly use a combination of explicit and inherited permissions (at the collection, site, library, and item level) to control access to resources. Other applications and services may use explicit app-defined permissions, rely on Azure Active Directory (AD) or local group permissions, or may use another access control mechanism altogether.
When moving to SharePoint Online, it's important to understand how these currently work so that you can plan for how permissions will work in the future. Many tools provide some sort of mapping between on-premises security controls and SharePoint Online security controls. This can be automated through the tool's interface or through the use of a separate mapping file.
For example...
