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 on hold. If at any point you attempt to withdraw coins that would make your account worth less than the amount on hold then the withdrawal attempt will be blocked.

Holds only affect withdrawals. You can freely trade without limitations. There are many reasons for a possible account hold such as returning after a long hiatus or your balance exceeding your VIP level's maximum withdrawal limit.