The phrase "yummy filling" means a tasty and satisfying substance or food that fills you up and makes you feel full or satisfied after eating. Full definition
As much as I loooove a good sandwich, one that's warm and toasty and packed with yummy fillings, I rarely ever have one because I'm just not a big lunch eater. (brighteyedbaker.com)
I like mine with lots of yummy fillings, so I love the fact this recipe has tofu steaks and a chickpea «scrambled egg». (eluxemagazine.com)