What is the essence of the Norman theory, what is in this theory based on actual facts?

The essence of the Norman theory is based on the fact that in the middle of the 9th century the Varangians came to the lands of the Slavs and helped them create their own state. If you look at the ancient historical document – “The Tale of Bygone Years”, it says that in 862 the Novgorodians called the Varangians Rurik, who since then began to rule in Novgorod and was actually the first ruler of the Rus state. This fact confirms the veracity of the Norman theory.