It means that after repeated practice, you have mastered the objective laws of things and can use them with ease.

Zhuangzi's Health Preservation Theory originated from Zhuang Zhou in the pre-Qin period;

"I know how to solve the problem for Wen. The touch of the hand, the leaning of the shoulder, the shoes of the foot, the place of the knee, and the knife are all alto. "

Translation:

There is a chef named Ding who specializes in slaughtering cattle. Wherever his hands touch, his shoulders rest, his feet step on, and his knees stand, he makes a sound of separation of flesh and blood, and the knife goes in, making a louder noise. All these sounds are not out of tune.

Simply put:

An expert who knows cows, a chef named an expert who knows cows, is a metaphor for a person who knows the objective laws of things. He is skilled, handy, and practice makes perfect.