Portal Accounts allows a user to authenticate themself to a portal, so that the portal can keep track of the data belonging to the user and keep it pinned to Skynet. Any Skynet portal can setup their own account parameters, but see the Skynet Guide's Portal Accounts page for more info on how Siasky.net handles user account.
For the most part, portal accounts work in the background, out of sight of the user or developer. Requests to the web portal will contain a cookie that is transporting an encrypted JSON Web Token (JWT) which authenticates requests.
For web applications, a user accesses the app through the portal they have an account with. Afterward, uploads and downloads automatically register as to the user for longer pinning and counting towards their storage quotas.
For server-side applications, the application itself will want a user account with a portal. Requests will need to contain the JWT to inform the portal to pin uploaded data.