What is meant when the nodes select the chain with the highest cumulated PoW? How is the highest highest PoW calculated? I thought the concept itself is called PoW.
PoW is being used in two different ways here. It is perhaps better to say "chain with the most work" instead of "highest PoW".
The valid chain with the most work is the chain that nodes choose as their main chain. The "most work" is quantified by summing the expected number of hashes required to mine each block in that chain. E.g. a chain which has 2 blocks with block 1 expecting 3 hashes to find and block 2 expecting 5 hashes to find would have a chain work of 8.