The horizontal SRAS curve captures the ideas that in the short run the price level is fixed and that firms are willing to supply any amount of output at that price. If the short run is a very short period of time, such as a day, this assumption is realistic. For instance, an ice cream store posts the price of ice cream in the morning and sells as much ice cream as is demanded at that price (up to its capacity to produce ice cream). During a single day, the owner typically won’t raise the price of ice cream if the quantity demanded is unusually high; nor does the owner lower the price of ice cream if the quantity demanded is unusually low.
The tendency of a producer to set a price for some time and then supply whatever is demanded at that price is represented by a horizontal SRAS curve.