Whale
A whale is a person or entity holding enough of a coin that their trades can move its price.
Large holders can push a thinly traded market with a single order, and their transfers to exchanges are watched as possible signs of selling. On-chain trackers publish big transfers as they happen.
Whale activity is easy to over-read: a coin moving between a holder's own wallets looks the same as a sale. Volume spikes on the chart, covered in the volume guide, are the part you can verify.