By locating proteins known as antigens on the surface of those cells, the immune system can identify foreign substances in the body. CAR T Cell Therapy, which are immune cells, have their own proteins called receptors that attach to foreign antigens and aid in causing other immune system components to start destroying the foreign object.
Antigens and immune receptors have a similar relationship to a lock and key. Each foreign antigen has a particular immune receptor that can bind to it, similar to how a lock can only be opened with the proper key. Cancer cells contain antigens as well, but if your immune cells lack the proper receptors, they won't be able to bind to the antigens and aid in the eradication of the cancer cells.