The European Social Charter is something like a list of promises that grown-ups (countries) make to their citizens about what they will do to make sure that people have a good life. When countries sign the Charter, they agree to do things like make laws to protect workers from unfair treatment, ensure everyone has access to good education and healthcare, and make sure that people have enough money to live on.
The Charter is a bit like parents promising to take care of their children. Just as parents want their children to grow up healthy and happy, countries promise to take care of their citizens and make sure they can live their best lives. When a country signs the Charter, they agree to follow its rules and try to do their best to take care of their people.