In PoS, forgers stake their ether on a block. These ether are locked and can't be used. What does it mean by locking of ether and how does it work?
In the existing Casper+Sharding spec, validators join by sending 32 ETH to a special smart contract on the current mainnet. That ETH is essentially destroyed. Then when the beacon chain sees these deposits, it mints the 32 ETH on the beacon chain and puts it in a lock box. The protocol will then increase/decrease their balance in the lock box based on the validator's actions.
The idea is that you must put your funds at risk in order to secure the network. If you perform maliciously, you will lose your funds. If you act logically, you will help secure the network.
The reason this works is because of the economic cost to behave in a negative way. In the current PoW model, you need 51% of the mining power to overtake the network. In PoS, you need 51% of the economic power to do so. This also helps with decentralization of the network.
There are other factors that help the decentralization and security, such as the limits on the amount on address can stake and various other limitations.