Hey and thanks for trying out the game!
If you downloaded the game, then there is a text file in the game folder called FishingTips.txt that explains and gives additional info about the fishing minigame.
But in case you're playing the web version, I'll copy the relevant section here as well: "After you cast the line with either the lure or a mix of seaweed & algae, a fish silhouette will soon appear near the cork. It can randomly pop up in 4 different directions: up, down, left, right. To coax a fish into biting you simply need to click on the fish shape to either slack or pull the rod in the correct direction (the text descriptions will help you here). After a few successful clicks the fish is hooked and you can reel it in. There are no real time limits to any of these actions, you can operate at your own leisurely pace and as long as you click in the right spots, you'll get your prize."