Units = the amount you want to buy/sell
Price = how much BTC you will pay for it. ( bid for buying / ask for selling ) if you press on the button you can choose market price from it .
Limit = the condition to buy/sell .
If you really do not know this basic then I really recommend you to not start trading right now take sometime learning the basics first .
That is a good advice, I think for new ones it is just better to learn at first and than try to trade with small ammounts, to see how all this system works.