While using a computer, all the data manipulated is written temporarily in 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.

After a computer is powered off, the data in RAM disappears rapidly, but it can remain in RAM up to several minutes after shutdown. An attacker having access to a computer before the data in RAM disappears completely could recover important data from your session.

This can be achieved using a technique called 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 rapidly.

As far as we know, cold boot attacks are not a common procedure for data recovery.

In a research report from 2011, Defense Research and Development Canada concluded that cold boot attacks can be useful in some cases to acquire data in memory but are not a panacea and have many drawbacks dictated by the laws of physics, which cannot be overcome by the technique. The authors recommend to only use cold boot attacks as a last resort when all other avenues have been exhausted.