CCR5 is the protein found on the surface of CD4 cells. HIV can enter a CD4 cell by attaching itself to this protein. CCR5-∆-32 (CCR5-delta-32) is a different (mutated) version of the CCR5 protein which makes it difficult for HIV to attach itself to the CD4 cell and to enter it.
Inheriting this mutation from both your parents (‘double mutation’) means you cannot become infected with HIV. Approximately 1% of people with mostley a North European background have the double mutation. Their stem cells have been used for the successful stem cell transplants which led to a number of people being cured of HIV.