If we assume the aperture on the 18-55mm is f/5.6 at 50mm, with the 50mm 'nifty fifty' right down/wide at f/1.8, then the prime will let in at least
8 times the amount of light that the 18-55mm will.
I read this
Tedious Explanation of Apertures when I first got my camera (actually, before) and it's a great explanation. I was sad enough to set up my own spreadsheet to do precise calculations.
50mm @ f/1.8 gives you an aperture area of 606mm square.
50mm @ f/5.6 gives you an aperture area of 63mm square.
Almost 10 times as fast. 9.67 to be precise. Hence the popularity.
Down at 50mm f/1.2 there is 1364mm square aperture.
I was also just reading about a 50mm lens that Stanley Kubrick used in Barry Lyndon, the NASA/Zeiss 50mm f/0.7. Originally developed by Nasa to view the dark side of the moon (!).
Even though it's just '1.1' difference, the aperture area is a collossal 4007mm square. Geektastic.