Volumes
An app's own files are replaced on every deploy. Anything that must last — uploads, generated files, a SQLite database — belongs on a volume. When you add a service, tick Keep files between deploys on the Configuration step (wdot suggests it when your app looks like it keeps a SQLite database). For an existing app, add one in Settings → Files and SFTP: give it a mount path inside the app (for example /data) and a size, then Save.
SFTP
Turn on SFTP in the same tab to reach a service's volumes from an SFTP client such as FileZilla or Cyberduck. wdot shows the host, port, username and password. Rotate password replaces the password; turning SFTP off closes access.
Server storage
A server's Storage settings show how much space each project uses. You can add more storage there; see the note in Create a server.