Centering refers to how well the printed image is positioned on the card — ideally, the borders on all four sides should be perfectly even. When the image shifts to one side during the printing or cutting process, the card is considered off-center, and that impacts its grade.
Grading companies measure centering as a ratio — 50/50 is perfect, meaning equal borders on both sides. A card that's 60/40 is slightly off; 70/30 or worse is noticeably misaligned. The further off-center a card is, the lower the grade it'll receive.
Centering is one of the four main criteria graders evaluate (alongside corners, edges, and surface). If you're submitting cards for grading, always eyeball the centering first — it's the easiest flaw to spot and one of the most common reasons cards miss that perfect 10.