Warren Buffett returns to tell us about his crypto experience so far
A whitepaper is a technical but relatively accessible description of how a piece of technology works.
The cryptocurrency community gets the term from the cryptography community, where a whitepaper is used to describe encryption functions and the like.
Here's an example for the blake2 cryptographic hash function.
The canonical example of a cryptocurrency whitepaper is the description of Bitcoin.
The issue with the crypto community is that non-technical investors (Lord, forgive us our redundancies) don't have the theoretical background to evaluate the merits of a whitepaper. They read a large LaTeX-rendered PDF, go "Yep, that's definitely math," and accept all claims made in the abstract as fact.
Cryptography whitepapers are scrutinized by large numbers of smart people who work for companies that cannot afford to get this stuff wrong. In contrast, the average ICO is scrutinized by 26-year-old Internet libertarians who took a linear algebra class in undergrad.
Also, note that just because a whitepaper exists does not mean that the technology itself actually exists. You can release whatever whitepaper you want; it doesn't mean that you wrote a program that implements it, that your program works, or that your program won't fail for an entirely different reason.