Getting unique values from a list of objects with a List<string> as a property

If I understand, you want a list of all of the unique certifications among all of the employees. This would be a job for SelectMany:

var uniqueCerts = empList.SelectMany(e => e.Certifications).Distinct().ToList();

You want to use SelectMany, which lets you select sublists, but returns them in a flattened form:

stringList = empList.SelectMany(emp => emp.Certifications).Distinct().ToList();