Well, kiddo, there was a time when a lot of countries were part of a group called the Soviet Union. This was because they shared the same ideas about how to run a country and they had a big leader called the Soviet Premier.
When the Soviet Premier was leading, he wanted all the countries in the Soviet Union to be happy and safe, so he made sure that they all got along and didn't fight with each other. This time of peace and cooperation between the countries in the Soviet Union was called pax sovietica.
Basically, pax sovietica means that all the countries in the Soviet Union were peaceful friends and didn't want to hurt each other. This was important so that everyone could enjoy a happy and calm life without any worries.