Complex sentence: IL-1RA, is an agent which binds to the same receptor on the cell surface as IL-1 (IL1R), and thus prevents IL-1 from sending a signal to that cell.
Split into simpler sentences: IL-1RA, is an agent which binds to the cell surface interleukin-1 receptor (IL-1R) which is the same receptor that binds interleukin 1 (IL-1). Hence IL-1RA prevents IL-1 from sending a signal to that cell.