Social equality is when certain goods and responsibilities are divided evenly across a society. Those goods can also be ideals, such as freedom.
Pretty much all societies in the history of the Earth have had unequal divisions. That is, they have been and are divided based on factors such as gender, race and class, creating unequal societies. Social equality activists work to bring about equality for all people in society so that these divisions do not create better opportunities for only a select few. Some of the "goods" that can be equally divided across society include education, health care and wealth. What goods make up social equality is a matter of debate.