Bait is an item that is used on wild Terra Monsters to lower their resistance bar, making them easier to catch with a Crystal.
There are three tiers of bait, with each tier lowering a certain amount of the resistance bar. Using Apples, Tomatoes and Oranges as an example, Apples are 'an effective bait for wild Earth creatures', this means that if you feed a wild Earth Terra Monster an Apple, its resistance bar will go down. However, if you feed it a Tomato instead, the bar will deplete even more. And if you feed it an Orange the bar will deplete even further. Feeding a Terra Monster a piece of bait that does not match it's typing will not lower the resistance bar as much as feeding it a piece of bait which is the same type as the Terra Monster you're trying to catch.
To use bait, simply click the 'Bait' icon in the battle screen, select your piece of bait and click on it, this will feed the bait to the Terra Monster.
Bait can also be used to feed your own Terra Monsters, replenishing their hunger bar and keeping them happy. This must be used in conjunction with Cleaning, otherwise your Terra Monster will not gain any evolution points. It does not matter what type of bait you feed your own Terra Monsters in your ranch, the hunger bar will fill up regardless of what food is used.
Notes: You cannot feed your own Terra Monsters Meat.