A blockchain is formed by having participants in the network, the nodes, pick what's the next transactions to bundle together in order to declare the next block. However, whoever picks the next block can effectively control what does and does not happen on the network. How do you make it so that block producers don't just write in whatever benefits themselves, like frontrunning every trade?

In proof-of-work blockchains like Bitcoin and Ethereum, the block producer is called a miner, which has to brute force search for a number that satisfies a condition based on the difficulty. In other words, keep trying numbers until you by pure luck stumble upon one that passes the difficulty test. This makes it so that the block producer is unlikely to have any connection with the transactions being written to the block, removing bias.

In proof-of-stake blockchains, validators are the block producers and need to follow some algorithm to come to a consensus on what goes into the next block. To discourage bad behavior by validators, each validator needs to provide a "stake". This "stake" is essentially a batch of cryptocurrency that signifies that the owner of this validator is a stakeholder in the wellbeing of the network. And that in the event that this stakeholder/validator misbehaves, the network is allowed to take away some or all of this staked cryptocurrency as a penalty.

How to go about staking depends on the network's rules. Here are a few examples:

1. In the case of Ethereum 2.0. Staking has to be done by 32 ETH deposits per validator. There are services such as Ankr and Lido which allow people with smaller wallets to pool together to make 32 ETH to run a validator. Whoever runs the actual validator will receive the funds, which can then be shared as seen fit. Keep in mind, Ethereum 2.0 staking doesn't allow withdrawals until months after the transition to proof-of-stake is complete.
2. In Polygon and Polkadot, both have a limit to the number of active validators. The active ones are picked from all online validators by the amount staked on them. The staking amount is determined by the amount contributed by the owner of the validator hardware as well as those who nominate or delegate to it. So to stake on these networks, simply buy the MATIC or DOT, and go through the process of picking a validator that you'll nominate. The network will lock your funds and if the validator becomes active, you'll receive a share of the rewards.
3. Algorand is a strange one. It's basically pays out to wallets based on how much is in the wallet. No locking, no slashing (penalties). I don't really understand how this works, but I read about it and it seemed bizzare.