Each trading day, you are charged or reimbursed an interest fee called a swap. The interest earned on long positions is paid on short holdings when trading on margin. Carry is the net interest difference, and carry traders want to profit from it.
Positive carry occurs when you receive more interest than you are obligated to pay, and the excess is deposited directly into your account. Negative carry amounts are deducted from the account. When a trade is opened and closed on the same day, there are no interest implications.