"Staking" is a way of maintaining the operation of a "blockchain network". This is done using the Proof of Stake (PoS) algorithm, which is explained in the tabs for cryptocurrency.
By maintaining the operation of a PoS "blockchain network", you will receive "rewards" when you "stake" a certain amount of the cryptocurrency. In a simple explained way, this means that you "lock off" part of the cryptocurrency you have in the network. One of the most well-known cryptocurrencies using the PoS algorithm to maintain the operation and security of its decentralized network is Ethereum.
With many cryptocurrency brokers, as well as directly in various "blockchain wallets", you can earn a percentage rate of return over time if you deposit a certain amount of your currency in a "staking pool".
You can easily think of "Proof of Stake" as getting interest on money you have in the bank. Different cryptocurrencies offer different degrees of income / interest rate for you to run their "blockchain".