The movement of a watch is like “behind the scenes” crew that makes the performance go on. You’ll probably never see it first hand, but it's what makes the difference between a watch that keeps time and a multi-generation heirloom. The most basic watch can be comprised of over 100 tiny parts while some in-house movements have over 1000.
Think of a watch movement like an engine in a car. The engine takes in fuel (in the case of a movement, it absorbs energy from motion or a battery) and makes the pistons fire, starting the engine and allowing the car move to move fast. When you have a watch with built with an in-house movement, it's as if you have a car built to outperform a Tesla Model S.
https://www.shreve.com/blog/why-it-matt ... -movement/