The CE mark is the manufacturer's declaration that a product meets European Union safety standards. It is not drone-specific — you see the CE sign on toys, appliances and electronics — but on a drone it sits right next to the C-class marking.
The two labels play different roles: CE says "this product is legal to sell in the EU", while the C-class label says "these drone rules apply to this aircraft". A class mark without CE (or the other way around) on a new EU-market drone is a red flag about where the product really came from.