Keys are currency in Ultimate Omelette and Cluck of the Dark Side, used to buy Unlockables. They are found in-game through destroying enemies with a yellow glow around them, which has a chance to happen roughly every minute. If Cheats have been used in a run, the Keys will no longer spawn. Once collected, they give 11111 points (in Rookie).
On average, unless you're planning to farm keys on purpose, you'll receive 50 Keys per mission.
In this game, you'll need a total of 3040 Keys to purchase everything. Unless you want to play through 60 missions, you'll have to farm them, and here there aren't any unlockables that help you collect more keys.
Cluck of the Dark Side
On average, unless you're planning to farm keys on purpose, you'll receive per mission 50 Keys in Rookie, 75 Keys in Veteran, and 100 Keys in Superstar Hero. You're able to make keys spawn more often (by 25%) if you're using the Gold Digger unlockable. The Scoring unlockables also help. By making the game harder, they will also cause you to collect more Keys because you'll spend more time on completing waves.
In this game, you'll need a total of 3710 Keys, which without the helpful unlockables would require 37 missions. Gold Digger brings the number down to 30, and Scoring unlockables even more.
The Keys can be spent on 31 different unlockables - 5 cosmetic, 5 spaceship related, 4 UI related, 5 difficulty related, 5 weapon related, 4 satellite related and 3 special paintjobs. You can read more about them here.