Carry is a term used to describe the overall cost (or benefit) from holding a position as a function of time. It is meant as a way of expressing the yield in a position. For long option positions and debit spreads, carry is negative because of theta (the time decay of an option’s price). 

The cost of carry is often evaluated in terms of time decay (negative theta). And for short options and credit spreads, the carry is positive, meaning that theta would be positive.