We  have to move from our devotion to independence, through an understanding of  interdependence, to a commitment to human solidarity. That challenge must find  its realization in the kind of community we build among us. Love implies  concern for all - especially the poor - and a continued search for those social  and economic structures that permit everyone to share in a community that is a  part of a redeemed creation (Rom 8:21-23). (United States Conference of Catholic Bishops, Economic Justice for All, no. 365)

The solidarity which binds all men together as members of a common family makes  it impossible for wealthy nations to look with indifference upon the hunger,  misery and poverty of other nations whose citizens are unable to enjoy even  elementary human rights. The nations of the world are becoming more and more dependent  on one another and it will not be possible to preserve a lasting peace so long  as glaring economic and social imbalances persist. (St. John XXIII, On Christianity and Social Progress [Mater et Magistra], no. 157)

Other Sources:

7 Themes of Catholic Social Teaching from the USCCB

Catholic Relief Services

Discussion Guide on Solidarity