The non-delegation doctrine is essentially the idea that Congress should make all the administrative laws rather than just the legal laws. So instead of relying on a government agency to carry out the law, Congress would tell the govt agency exactly how to carry out the law.
This non-delegation doctrine is bunk. Congress has been delegating powers of governance since its founding: https://www.theatlantic.com/ideas/archive/2020/05/nondelegation-doctrine-orliginalism/612013/. There is no historical evidence to support the idea. More to the point, this idea of non-delegation arose at the same time as the administrative/welfare state.
So it is less a constitutional theory and more a reaction by ideologues for things they disagree with.