It's not that hard. A repeating decimal is kinda like pi. Except rather than there being a non-repeating string of numbers, you have one that repeats itself. So for example one ninth is 0.1111111, but that 1 repeats infinitely. There are also ones like 1.832323232 where the 3 and 2 repeat infinitely. So if you have 0.999999..., then that number is infinite zeroes and then a 1 away from becoming 1. And because of that, it's so infinitely close to 1 that in absolutely any circumstance ever in any practical calculation, from speed in a car to quantum physics, it can always be expressed as just 1.