These are investors who hold a huge amount of cryptocurrencies. This includes trading thousands of Bitcoin or millions of Altcoin, for example.
There is often a lot of talk and speculation about whales manipulating the market for their own profit. This includes things like building a wall at certain prices to prevent people from selling above that price. And selling a large volume of a currency in a short period of time to drive the price down, then triggering people's stop-losses which exacerbate the problem and cause further downside action, at which point the whales buy back in cheaper.
