Staking is the purchase of cryptocoins and keeping (holding) them in a cryptocurrency wallet for a particular period of time. This is similar to a fixed deposit in the fiat currency world which rewards you with a fixed interest rate at the end of the stipulated time in the contract.
This brings us to the concept of Proof of Staking (PoS). It is an agreement algorithm for some cryptocurrencies which creates new blocks that you can add to the blockchain. These created blocks are actually staked by somebody who is holding some cryptocoins already and helps to validate a new deal on the platform.
Source: https://medium.com/kucoinexchange/staking-high-yield-and-risk-free-oh-you-wish-a339de452a6d