A "Hold" is a restriction in your account that prevents you from withdrawing some or all of your coins.
When you have a hold, we will make sure your account is worth at least the amount you have in hold. If at any point you attempt to withdraw coins that would make your account be worth less than the amount in hold then the withdrawal attempt will be blocked. Keep in mind that only a few coins are considered to calculate the valuation of your account: BCH, BTC, DASH, ETH, LTC, TUSD, USD, USDT and XMR. The rest of your coins are worth $0 for valuation purposes.
Holds only affect withdrawals. You can freely trade without limitations.