Main account is designed for external operations (such as depositing and withdrawing) and storing your funds.
Trading account is used for placing orders. All coins which you buy and sell go from and to your trading account.
To be able to place orders, transfer your funds from your main account to your trading account. Please proceed to How to perform a trade for step-by-step instructions.
In case you forget to transfer your funds to trading account, you will receive "Insufficient funds" error when trying to place an order. Check the article Insufficient funds while trading.
The same error will show up in case you fail to transfer your coins to the main account before trying to withdraw them from the exchange. Here is all the information about this issue: Insufficient funds while withdrawing.