A whale is a person that have plenty of a particular currency. For example a bitcoin whale is a person that have alot of bitcoin. It must be 1000btc and above to qualify as a whale. Price can be manipulated my a whale when they dump some of their bag which make people to think that the price is dropping and cause people to panic sell and they will buy back the dip and acquire more btc. The can also pump the price by buying more of btc for example and people will think the price is pumping and they will FOMO ( fear of missing Out) the whale will dump on them and take profit.
That is a very sound and logical explanation. So now I know what a whale is by definition in the market.
When you really have the right number of resources at your disposal then you can really manipulate the market.
Given that, they have the capacity to predict how the market behaves.