The noun fluctuation refers to the deviations along the path from one point to another. We see frequent fluctuations in the stock market, as prices go up or down, and also in the weather, which is always changing.
The original form of the word fluctuation appeared as a mid-15th-century French word derived from the Latin fluctuationem, meaning "vacillation." The verb stem fluctuāre meant "to undulate," referring to waves, giving rise to the idea of change and movement that is at the heart of the word. It might help to remember that fluctuation has the same root as the word fluent, and to think of it as something flowing, with frequent change along the way.