The use of a persistent volume in a system which is designed to provide anonymity and leave no trace is a complicated issue.
Read carefully the warning section.

Enable the persistent volume

  1. When starting Tails, in the Use persistence? dialog of Tails Greeter, choose Yes to enable the persistent volume for the current working session.

  2. Enter the passphrase of the persistent volume in the Passphrase text box.

  3. If you select the Read-Only check box, the content of persistent volume will be available and you will be able to modify it but the changes will not be saved.

Use the persistent volume

To open the Persistent folder and access your personal files and working documents, choose Places ▸ Persistent.

For advanced users, to access the internal content of the persistent volume choose Places ▸ Computer, and open the folders live ▸ persistence ▸ TailsData_unlocked.