During lent, you weren't supposed to eat meat as part of your fast. However, not eating meat is... not as enjoyable as eating meat, so they basically declared that fish doesn't count as meat so they could eat it without breaking the fast.
For similar reasons, they also declared beavers to be fish later on.
Except that's not how it happened. Lent was someting that took several centuries to occur in any form, and what was allowed and what was prohibited varied greatly for long after. Fish and shellfish from the start were allowed more often than things like butter, oil, or wine.
During lent, you weren't supposed to eat meat as part of your fast. However, not eating meat is... not as enjoyable as eating meat, so they basically declared that fish doesn't count as meat so they could eat it without breaking the fast.
For similar reasons, they also declared beavers to be fish later on.