This part nearly doubles the capacity of your master cylinder. Simply unscrew the flat cover on your master cylinder and replace it with this extra capacity lid. It is a particularly important part for disc brake cars because as brake pads wear, the pistons in your calipers move further out (to compensate) and the system needs more fluid to stay full. With this expanded cap you'll keep ample fluid in the reservoir as the brake pads wear.
These lids come with a new cork gasket. Note, these gaskets are prone to weep. Silicone fluid is good option when switching to an expanded capacity reservoir to reduce the risk of any paint damage to your master cylinder tray, which might otherwise be damaged by conventional brake fluid leaks. If you do switch to silicone fluid, be sure to first completely purge all conventional fluid from your system, as the two fluids are not compatible.