Jargon which is specific to a certain craft may not be immediately apparent to people who are not used to "talking the language."
The term "runnings" comes from the act of draining or 'running' the wort from the grain bed.
'First runnings" is the wort which is drained from the main mash. Generally, unless you are doing a full volume brew in a bag mash, you will have a volume of water for your strike water and mashing the grains. It is typically higher in sugar content
This then helps separate this wort from "second runnings' which come from the sparge water added to the grains to rinse off the residual sugars. Most brewers who fly sparge watch the gravity and pH of the second runnings to avoid drawing unwanted flavors from the grains.
I hope which helps. if not let me know what I can clarify!