The Umbrella is a key item that was introduced in Alpha 4.
In Alpha 4, there are five umbrellas scattered around the house. In future versions however, there are only two, and they require quite a bit more effort to find. There is also an umbrella in the Neighbor's basement, and it is the primary method of getting through the basement.
When opened, an umbrella acts as a parachute and slows the player's descent, allowing them to glide over wide gaps. It also prevents them from taking fall damage.