For example, assume that there is a investor holding a call option who sells it for $2.50. Immediately following this sale, the investor buys another call option on the same underlying security for $2.65. The debit spread is $0.15, which results in a loss of $15 ($0.15 * 100).
Although there is an initial loss on the transaction, the investor is betting that there will be a significant change in the price of the underlying security, making the purchased option more valuable in the future.