There are different ideologies that can be called "radical," but the one most commonly thought of as radical is communism. The communist view of social and economic justice is one that is similar to, but much more extreme than, the liberal view.
Communists believe that everyone should essentially be equal. They do not think that it is acceptable to have some people be much richer than others even if everyone has all the basic needs of life. Instead, they insist on equality.
In their pursuit of equality, they want the people who (in their minds) create the wealth to be the ones who benefit from it. For this reason, they do not want a class of people who own companies and make money from those companies while others do the work. To communists, this is exploitation of the workers.
Communists, then, believe that social and economic justice are achieved when everyone has essentially the same amount of material possessions and no one gets rich off of the work of other people.