To accurately measure the size or location of something requires energy. The more precise the measurement, the more energy is required. The amount of energy required get the precision below the Planck length would literally create a black hole.

I've never found a good link, and I'm not certain that I know best, but I can try to explain it to you.

First: an understanding of the Pauli exclusion principle. Often people ask "Why can't there be 3 electrons in that orbital, there's plenty of space?" The thing is that the electrons are completely¹ defined by just 4 numbers: spin (±½), shell (positive integer), subshell (integer from 0 to shell-1) and magnetic (integer form -subshell to +subshell). Why there can't be more than 2 electrons in the 1st shell is that you can chose spin from (±½), shell is 1, subshell has to be 0, magnetic has to be 0. Its like asking "Why can't there be 3 integers between 0 and 3, there's plenty of space?" and the answer is that whatever integer you come up with will be one of the 2 already known (1, 2).

Similarly, as I understand it, the fundamental laws of physics don't distinguish between "things" closer than 1 Planck length apart. That doesn't necessarily mean that the universe operates on a 1 Planck length grid, just that any two "things" separated by less than a Planck length are indistinguishable from one new "thing" with different properties.

I'm fairly confident in the PEP description, the Planck length one I'm less 100% sure about, but its how I understand it at least.

¹assuming a universe comprised of only a single hydrogen atom, otherwise the states of everything else in the universe can perterb the state functions and things can get messy, but usually not enough to merge shells.

I think the best way to understand the Planck length is to understand how we came to Planck's constant in the first place. Science Physics girl has a good video on it (she shared today that she's getting better, which is fantastic news)