What exactly are the "Windows Automatic Updates"?

like, what do they really do? what do they add, change, or... WHATEVER! i just wanna know the point... its just that it takes forever for my comp to start up, and the automatic updates wants to restart my computer... the "Restart Later" on was gray, and so was the red x in the corner of the box... i went into task manager to end the auto thingy, then... 5 SECONDS LATER... it shows up again, continuing the countdown... so... wtf? what makes it so important that it has to restart the computer? does it even really do anything at all?