Stakers can be in any one of the three states: validating, nominating, or chilling. When a staker wants to temporarily pause their active engagement in staking but does not want to unbond their funds, they can choose to "chill" their involvement and keep their funds staked.
An account can step back from participating in active staking by clicking "Stop" under the Network > Staking > Account actions page in PolkadotJS Apps or by calling the chill extrinsic in the staking pallet. When an account chooses to chill, it becomes inactive in the next era. The call must be signed by the controller account, not the stash.
PRIMER ON STASH AND CONTROLLER ACCOUNTS
If you need a refresher on the different responsibilities of the stash and controller account when staking, take a look at the accounts section in the general staking guide.
When you chill after being a nominator, your nominations will be reset. This means that when you decide to start nominating again you will need to select validators to nominate once again. These can be the same validators if you prefer, or, a completely new set. Just be aware - your nominations will not persist across chills.
Your nominator will remain bonded when it is chilled. When you are ready to nominate again, you will not need to go through the whole process of bonding again, rather, you will issue a new nominate call that specifies the new targets to nominate.
When you voluntarily chill after being a validator, your nominators will not automatically go away. As long as your nominators make no action, you will still have the nominations when you choose to become an active validator once again. However, if your nominators decide to nominate other validators then these nominations will take priority when the validator comes back. It may also be the case that your nominators change their entire nomination targets (all 16 of the allowed nominations). In this case, your nominators would need to explicitly specify your validator as a target when your validator comes back.
When you become an active validator you will also need to reset your validator preferences (commission, etc.). These can be configured as the same values that were set previously or something different.
If a validator was unresponsive or found to have committed a slashable offense within two eras, the validator will be removed from the active set in a process known as involuntary chilling. When a validator has been involuntarily chilled, it is necessary for the nominators that were previously nominating that validator to re-issue the nominate call.
Nominators who have the option to renominate an involuntarily chilled validator will have a display row to do so using Polkadot-JS Apps. This row is displayed in the "Account Actions" tab for the nominator under a heading that says "Renomination required". If your validator has been involuntarily chilled, you will need to request your nominators to re-issue the nominate call in order to start nominating you again.
An unbounded and unlimited number of nominators and validators in Polkadot's NPoS is just not possible due to constraints in the runtime. As a result, multiple checks are incorporated to keep the size of staking system manageable, like mandating minimum active bond requirements for both nominators and validators. When these requirements are modified through on-chain governance, they can be enforced only on the accounts that newly call nominate or validate after the update. The changes to the bonding parameters would not automatically chill the active accounts on-chain which do not meet the requirements.
For instance, let us consider a scenario where the minimum staking requirement for nominators is changed from 80 DOTs to 120 DOTs. An account that was actively nominating with 80 DOTs before this update would still keep receiving staking rewards. To handle this corner case, the chill_other extrinsic was incorporated which also helps to keep things backwards compatible and safe. The chill_other extrinsic is permissionless and any third party user can target it on an account where the minimum active bond is not satisfied, and chill that account. The list of addresses of all the active validators and their nominators can be viewed by running validator stats script.
