In crypto slang, a whale is a person or organisation that holds a very large amount of a particular cryptocurrency. There is no exact threshold, but the term is used for holders big enough that their activity could potentially influence the market for that asset.
Whales draw attention because a single large trade can affect price, especially in smaller or less liquid markets. Because many blockchains are public, some people watch large wallets to see when significant amounts move, treating it as a point of interest.
For beginners, the concept mainly explains a term you will hear often. It is worth knowing that large holders exist and can have outsized effects in thin markets, without reading too much into any single movement, which can have many explanations.