This is where the 9 hazels trees of Wisdom dropped nuts into the pool that the Salmon fed on.
Allegorically we could consider the 5 streams to be the 5 senses, maybe this represent the search for wisdom via experience?
We would then need to interpret what the 9 hazel trees represent. Within the Germanic world there were 9 realms or worlds. Within the Celtic world we have the 3 realms of Earth, Sea and Sky plus (arguably) the 3 realms in the Circles of Abred. The 9 trees could come from a 3x3 and so maybe the 9 hazels represent the sum of all knowledge across all the realms?
The Salmon, by eating of the nuts, acquires the sum of all knowledge. Maybe the search for the salmon is the method of inquiry?
In this respect it would be how Fionn accumulates such Druidic knowledge and wisdom - through experience, knowledge and inquiry.
The Salmon is a special, magical being because it starts in a small inland stream and then travels, across the ocean, far to the west (if you live in north-western Europe) and perhaps to the location of the Summer Lands. It then returns older and wiser ready to start the next generation.
Funnily enough down here in Sussex we don't have Salmon but do have the Salmon-trout which is a Brown trout that spawns and spends it's youth in the freshwater rivers, when it matures it migrates down to the estuary and then spends most of it's time living in the sea. Sometimes they migrate far upstream during the winter floods and then get trapped in pools, the wider stretches of the stream as the floods subside. I was reflecting on this as I passed some of our local Hazel trees that are laden with nuts.
As with all of the Celtic stories the allegory seems very obscure and tenuous but there is also the thought that the reason why the Druids were against writing their esoteric knowledge down was to encourage each generation to hunt for wisdom, knowledge and truth. That way it remains alive and reflects the needs of each generation and does not become Dogma from being written down.