What is the difference between 'Mark invalid' and 'Non-actionable'?

Both of these are a part of response actions, which can be accessed via the Response actions bar, which is displayed while viewing a response.

Marking a response as invalid excludes it from any reports and analytics for your survey. This can be an effective way to filter out spammy responses.

Marking a response as 'Non-actionable' does not remove it from any analytics or reports. This action is meant for those cases where the customer, when asked for their thoughts or opinions has left non-actionable comments like 'good.' which cannot be acted upon.