While using a computer, all the data manipulated is written temporarily in [[!wikipedia Random-access_memory desc="RAM"]]: texts, saved files, but also passwords and encryption keys. The more recent the activity, the more likely it is for the data to still be in RAM.
This can be achieved using a technique called [[!wikipedia cold boot attack]]. To prevent such attacks, the data in RAM is overwritten by random data when you shut down Tails.
Moreover, an attacker having physical access to the computer *while Tails is running* can recover data from RAM as well. To avoid that, learn the different methods to [[shutdown Tails|doc/first_steps/shutdown]] rapidly.