Bait – Stardew Valley Wiki

Bait can be attached to a Fiberglass Rod or an Iridium Rod, or inserted into a Crab Pot. Fishing rods do not require bait, but fish will bite faster with bait; specialized baits provide additional benefits. Crab Pots require bait to catch fish (unless the player has the Luremaster profession), but the type of bait has no effect on crab pots.

https://stardewvalleywiki.com/Bait