Encouragement, prompt to take action.
I grew up with this word in the sense of a 'pick-me-up', something that would jar a weary system into action. A fillip can be an action, a good cup of coffee, cuppa tea or an encuoraging word or action from another. It was also, as I recall, the name of a garden or houseplant fertiliser, whose goal was the same; to inject life into a flagging plant.
Merriam-Webster notes that a use of the word is "To encourage". That's a good way of putting it. I strongly suspect the etymology comes from an exagerrated pronuciation of the word 'flip', as in a swift, sharp blow. It seems to be from Germanic words with onomatopeic origin, as with flip, flick, fling, , implying movement.