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

When starting Tails, in the Encrypted Persistent Storage section of Tails Greeter, enter your passphrase and click Unlock.

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.