If you got here looking for hand weights (common mistake), see our dumbbells.
Olympic barbell weight sets typically have a 7ft barbell and a set of weight plates to load it with.
300 lbs may seem like a lot of weight for a beginner, but it's more about the variety of weight plate sizes for you to use. If you're lifting 135 lbs, for example, you will be using the two 45 lb plates, the largest size.
The total weight of these sets includes the barbell itself, where applicable. The 7ft bar included with most sets weighs 45 lbs, making the plates total 255 lbs.
These are called "olympic" weights in contrast to "standard" plates that have 1" holes. All of these have approximate 2" holes to fit on olympic bars.