A major part of the work of the Beacon Chain is maintaining the set of validators, which is the set of nodes that have placed the required stake of 32 Ether, who are responsible for running the Ethereum 2.0 system. There are several statuses that a validator can have, but only those marked as “active” take part in the Ethereum 2.0 protocol.
Nodes join the validator set by sending their stake to a contract on the Proof-of-Work chain (the current Mainnet). After some validity checks, the stake is locked up and the contract emits a log entry (an “event” in Solidity) which can be picked up by Beacon Chain clients. The node is then inducted into the validator set on the Beacon Chain.
Once active, validators participate in the protocol by proposing blocks, when chosen to do so, on both the Beacon Chain and, once they have been implemented, the shard chains. They also join committees that vote for blocks, as described below.
Validators can signal that they wish to exit the system and cease being involved. After a period of time (currently 97 days, but may be made more flexible), their stakes, plus rewards, minus penalties, will be returned into one of the shard chains. It will not be possible to unlock the initial stake on the PoW Mainnet, unless, perhaps, the whole system were to fail and the community agrees to a fork that refunds stakers. All of this is managed by the Beacon Chain.