This post is about the European Court of Justice (ECJ) and not the European Court of Human Rights (ECHR). The ECJ is part of the European Union, the ECHR is not. The ECJ is more correctly known as the Court of Justice of the European Union (CJEU).
Some UK citizens are concerned that the CJEU can overrule UK courts which they believe leads to a loss of sovereignty. The CJEU’s role is “Ensuring EU law is interpreted and applied the same in every EU country; ensuring countries and EU institutions abide by EU law.” UK Members of European Parliament vote on EU laws and only rarely object. The court has two judges from each member country. The court can overrule UK policy, so this does suggest some loss of national sovereignty.
Bear in mind, though, that the only way for a nation state to have complete sovereignty is to be completely isolated. Even the largely isolated North Korea is dependant on China.
Every trade agreement between two nations or trading block will have an arbitration clause: what happens if one partner believes the other is not following the letter of the agreement. If, for example, UK made a trade agreement with USA, the arbitrator will not be a British court, and so a little sovereignty is lost.
Jim Grace posted a list on Twitter of the times the Court overruled UK. You might enjoy reading through this list looking for something that has a negative impact on you personbally.