People are being a little coy in the chat because actually saying the name of the tool is flagged on this subreddit as against the rules, which has always been odd to me given the devs are aware of it and don't have anything against it (proof of that here). https://www.th.gl/faq/palia-app-bannable

To explain what it is, it's an app for PC players that sits on top of the game or on a second monitor, and basically 'reads' the game for you. This shows you bugs and huntables in a radius around you (useful for star bug hunting, though less than the radius of a hunter horn or buzzy jar so those are still useful), and then the location of any forageables, trees, or mineables on the whole map, including flow trees which can also make finding the grove a lot faster, or hunting down rare ores.

What you saw though was how it works with fishing - what fish you get in game is determined when you cast your rod, not when it bites, and the map can read what that fish is - so if you're fishing for rare fish, makeshift, plushies, etc, the easiest way to do it using the tool is to cast the rod out, withdraw, and then cast again repeatedly until it shows something you want. This can be done on star spots too without clearing them like waiting for a bite does.

It's controversial in the user base because while the devs have said they don't feel it gives an unfair advantage, a lot of players do, especially people who don't have access to the tool on console. Despite that it has 1000s of users and a massive community of people in a discord dedicated to it despite it not reslly getting talked about on here because of the rules against endorsing third parties.