0.999... is a way of writing a number that is very close to 1. It is like a shortcut that tells us that the number is almost exactly 1, but not quite. In fact, it's so close to 1 that if you keep adding 9s to the end, it would still never be exactly one. So if someone asks you what 0.999... is, it means that it is a number that is really close to 1, but not quite.