Brooks' law is a claim about software project management according to which "adding manpower to a late software project makes it later"
100% true, the reasoning behind this is really simple and I've experienced it first hand.
My game for example. I've done the majority of the work and then brought in another developer down the road. I know the code and what I changed, what does what, and where to find it. The new developer doesn't know this. They have to basically learn the code themselves, learn what does what, and learn what is found where before they can begin to start making their own changes to it.