The word "slum" here is important. A slum landlord would be someone who owns decrepit houses or apartments, and leases them to (typically poor) renters. The term is often shortened to "slumlord."
Shaw's socialist ("radical reformist") philosophies motivated him to show the terrible conditions the renters lived in, and to condemn the landowners for not providing better living conditions to the renters.
I'll disagree with roi g here; it's not so much leasing land to cultivators, but rather leasing houses to renters to live in.