A sky bottle is a bottle that has nothing but sky in it. It's an empty bottle.

The reading is a combination of kun'yomi and on'yomi readings. You learned this reading for 空 when you learned 空き缶. If you can remember that, you can remember this too! Oh and sometimes you'll see this written 空瓶 without the kana too. And when that happens it can be read からびん! So if you learned it one way and not the other, don't worry, we'll take both here. Just know that seeing it as 空き瓶 (and pronouncing it あきびん) is way more common.