This is an important building at it is meant for hunters who did not reincarnate up to the current difficulty , can train to become stronger. If you have upgraded from Easy to Normal level, and you don't have the Training Ground yet, then this should be on the top of your shopping list for things to construct. It is not so expensive at just 4800 Gold.
At the training ground you can use items left over from the previous difficulty level which are not of much use nor to produce training tactics books which will super speed up the levels of hunters that did not reincarnate during the difficulty level up.
If a hunter needs training, there will be a training icon available when you tap on him to interact with him. Until your hunters have trained to level 100 and can reincarnate they will not enter the hunting grounds.
It takes 30 minutes to complete the training session, but multiple hunters can train at the same time. The recommendation is that as soon as you increase your difficulty level, get all of the hunters that need training, into the Training Ground.
Training Ground - Training Multiple HuntersWhat do I need leftover orange for?
It doesn't seem like it is needed for much, but leftover orange and other items are utilized to produce Tactics Books to use on heros to level them up quickly after you have increased the difficulty level. It also helps to purchase these items from your hunters so they have room to pick up other things.
Using Spare Resources from Previous Difficulty Level