Christ died on a Friday. During Lent especially, Fridays are 'no meat days' because meat was considered an expensive and luxurious item. Many abstain from eating it and donate the money saved to the poor.
Fish is a way of avoiding meat, as it is seafood and many Catholic orders consider it as not meat, and used to be much cheaper.
This non-meat Friday tradition serves as a reminder of what Christ did for us when we eat.