50 F1.8 is a lens, it has electronics parts. And it allows you to use it when you mount it to a camera body, where the electronic parts will give you Auto Focus, etc... Every lens has a minimium focusing distance (MFD), where if the distance from subject to the camera sensor is any closer than the MFD it will not be able to focus. For this example let's just assume the 50 F1.8 has a MFD of 1m (making this up for this example)
A Raynox 250 on the other hand is a attachment you put in front of a lens. It's made up of glass elements but is not a lens. Essentially it's a Diopter, a +8 Diopter. Call it a magnifying glass if you like. It doesn't have any electronic parts and you don't mount it to a camera body. But when you attach it to the front of the lens, it reduces the focal distance to that which is less than the MFD. So in this case, since it's a +8 Diopter, you will need to object to be at least 12.5 cm (1/8) infront of your Raynox. Simply put, since now the object is alot closer than before from your camera sensor (12.5cm Vs 1m), the image captured is 'magnified'.
And this is the objective of macro photography - to capture things which are relatively small sized so that it 'appears' relatively big on your sensor.