A Slashing event is when a Validator is penalized for misbehaving within the Node Network. This could include extended periods of downtime, creating duplicate blocks, validating untrue/inaccurate blocks, participating in a coordinated attack on the network etc.
These types of events can often occur without malicious intent, so in many cases - especially when consequences are negligible or non-existent - a Slashing event may be applied (so there is a record of misbehavior) but the penalty for the slash is zero ( 0 ) so no tokens are actually lost.
One example would be if a node experiences an extended period of downtime. It may have a Slash of 0% applied (though more often, they will be Chilled). A slash greater than 0% wouldn't be applied until 10% or more of the node network goes down simultaneously.
In most cases where Slashing results in an actual penalty, that penalty is likely to be a small fraction of the staked amount, less than 1%.
More significant events, like a coordinated attack on the network where one or more Validators attempt to harm the network (e.g. by validating malicious blocks) can result in any amount up to 100% of the staked balance being slashed. This is reserved only for extreme and/or highly consequential events.
Slashes are applied to the misbehaving Validator's entire staked balance during the Era in which it occurred, which includes the Validator's own stake and the stake of all Nominators supporting that Validator in that Era.
If a slash is applied to one of your Nominated validators, but you are not Actively supporting that Validator during the offending Era, your stake will not be slashed.