A prime lens is a lens that has a fixed focal length, meaning you can't zoom in or out. The plus sides to primes are they generally have better image quality, are smaller and lighter, are generally faster, and cost less than their zoom counter parts. Keep in mind high end zooms can compete with primes for image quality. When people here refer to "fast" lenses, we are referring to the maximun aperture of the lens which determines how much light the lens can let in. The lower the number the better as they are actually fractions such as 1/2.8, 1/4 so f/2.8 is larger than f/4. The max aperture is the number listed after the focal length on the lenses (i.e. 70-300mm has a max aperture of f/4 on the short end and f/5.6 on the long end) and lenses with a max aperture of 2.8 or lower are considered fast lenses.
Now for low light sports you need a fast, longer lens to keep up shutter speeds and have enough reach for a decent picture. Fast and long is an expensive recipe. To get similar reach to what you Pro1 has, you'd need about 135mm of focal legnth, but for sports longer would be better 200mm would be the shortest I would personally use. At 135mm you could pick up a Canon 135mm f/2.8 soft focus lens, just switch off the soft focus which would run you just under $300 new. At 200mm you can look at the Canon 200 f/2.8L, not cheap at over $600 new, probably around $550 for a mint used copy but is just a fantastic lens all around. A used Sigma 70-200 2.8 could also do you well, probably around $600-$700 used.
I'm taking pictures of everything, so go ahead and take this place away from me.
5DII, 16-35mm f/2.8L II, ZE 28mm f/2, 35mm f/1.4L, ZE 50mm f/1.4, 100mm f/2.8L IS, 580 EX