If we are talking about modern liberalism (such as the Democratic Party is connected with), social and economic justice includes more equality among people and a guarantee that everyone will have their basic needs met.
To liberals, it is important that everyone should get their share of the wealth and the opportunities that our country produces. Liberals, for example, are likely to express such opinions as the opinion that "it is wrong for x number of people to lack health care in the richest country in the world." They tend to look to the government to provide what they see as fairness and to make sure that everyone has some basic standard of living.
This is in contrast to conservatives who feel that the role of the government is to get out of the way of the economy. To conservatives, economic and social justice is achieved by letting the economy work, unburdened by government regulation. This is, they believe the most just system and the one which will lead to more prosperity.