Horizontal spread involves options with the same strike, same type but different expiry dates. This is also known as calendar spread or time spread. The underlying reasoning for executing and participating in horizontal spreads is that these two options would have different time values and the trader believes that difference between the time values of these two options would widen or shrink. This is actually a strategic play on premium difference between two options prices squeezing or widening.
If the implied volatility on the near-term option is higher than the IV for the far-dated option and also one is expecting the volatility to fall over the term suggests that one should sell the near-term expiry option and buy the far-expiry option; of the same underlying.