Whales are defined as people or organizations that have lots of money to buy and sell huge quantities of an asset, such as cryptocurrency. Their ability to move large amounts gives them the potential to manipulate prices.
Because whales have so much money, when they buy or sell an asset, they directly and/or indirectly affect the price.
For example, when a whale sells huge amounts of bitcoin, the price may start dropping. This would be a direct change. Investors who are not confident in bitcoin will see the drop in value and also sell. By scaring uncertain investors and getting them to sell, a whale can cause the price of bitcoin to make large, fast drops.
The same works in the opposite direction. When a whale buys a huge amount of bitcoin, the price may start increasing. Investors who want to make quick money will see the huge amounts of money coming in and also start buying. Now the price of bitcoin will shoot way up.