Slashing is a scary word. But what exactly is it, how can it happen and how worried should you be?
TLDR: Realistically, the only condition that can cause a slashing event is if you run your validator's keys on two nodes at the same time (such as a failover / redundancy setup, where your backup node accidentally turns on while your main node is still running). Don't let this happen, and you won't get slashed.
Slashing cannot occur from being offline for maintenance.
Slashing is a term used to describe the response of the Ethereum network to a validator acting against the rules of the network. Validators perform a number of duties (e.g. attestations and proposing blocks).
If someone wanted to attack the Ethereum network they could propose multiple blocks or attest to multiple conflicting blocks. To disincentivize attacks on the network, in a Proof of Stake (PoS) system, validators have something at stake, which is currently 32 ETH per validator. When a validator breaks the rules of the network, two things will happen:
The validator has some amount of ETH taken from that initial 32 ETH staked balance.
The validator is force exited and removed from the validator pool.
The amount of ETH taken as a penalty varies on the state of the network. If a small number of validators are slashed simultaneously, then a rough estimate of the slashing penalty is 1 or 2 ETH. In an incredibly rare Black Swan event, when a large portion of the network is simultaneously offline or breaking the rules (e.g. in a coordinated attack) then the slashing penalty can be up to and including 100% of the stake.
When your validator is force exited and the stake is withdrawn you are able to re-stake your remaining ETH (if you still have the 32 required), after going through both the exit queue and activation queue again.
