Social justice is the idea that everyone in a society should be treated fairly and have equal access to its benefits. If you care about people having enough food and somewhere to live, you care about social justice.
Justice is fairness, and social justice simply puts this concept in the specific context of a society. Without social justice, there is no protection against discrimination on the basis of race, religion, ability, gender, or sexual orientation. Social justice means equal access to opportunities, as well as basic necessities like health care, shelter, clean water, and a healthy environment.