If they don’t mine or produce blocks, why do some people still run their own nodes?
One detail: light nodes download only block headers. When they need data for a particular transaction, they request the data and a Merkle proof from a full node.
Besides running a node, validators must propose blocks and vote according to the rules. Being offline for a long time can result in part of their balance being deducted, while misconduct can lead to their stake being slashed.
Understanding this matters more than memorizing a bunch of terminology. Understanding the rules helps you make more clear-headed decisions; it’s not for predicting market trends.
On Ethereum, becoming a validator requires staking at least 32 $ETH . The current price is 2014.15.

Don’t treat this as a buy or sell signal.