“Easy peasy” means “very easy” (e.g., “That math test was easy peasy”).
It’s a reduplicative (repetition of sounds) of the word easy. “Easy peasy” is an informal expression often used by or with children. A variation is “easy peasy lemon squeezy,” which further accentuates the expression’s rhyme.
A similar expression is piece of cake, though this isn’t as informal as “easy peasy.”
QuillBot’s free Paraphraser can help you decide in which contexts it’s appropriate to use expressions like these.