If you can deal with the sub-par production quality, this is a great primer (IIRC):

https://youtu.be/GHgi6E1ECgo?si=uWSJMxSF2SLEulNZ

My recollection of the argument, drastically paraphrased:

1) The most amount of “stuff” or “information” that can fit in a given volume is a black hole

2) adding more “stuff”/“information” will increase the size of the black hole

3) the counter-intuitive but mathematically sound part:  in our everyday experience, we expect “stuff”/“information” to correlate to VOLUME.  In black holes, the correlation is to SURFACE AREA.

Ergo:

For any volume of space, the entirety of the “stuff”/“information” can be encoded with the surface area of the region.