This happens because you have the "Show Desktop" trackpad gesture turned on. The gesture that activated it was to spread your thumb and three fingers. You can undo it by pinching your thumb and three fingers together.
Look under the "More Gestures" tab in the "Trackpad" section of the System Preferences for a video demonstration of how to do it, or scroll down to the "Show Desktop" section of this article: https://support.apple.com/en-us/HT204895
This is usually what the child/pet/alien has done to get the system in that state in the first place.