It usually refers to a camera/lens auto focusing error. When you AF specifically on a certain subject, or a small part of it with the centre point AF, the camera locks on, indicating sharp focus. But when you look at the result, the sharpest point is slightly in front of where you intended it to be.
It often turns out to be a user error rather than an equipment fault. Bear in mind that the area the AF point works within is usually quite a bit larger than that indicated in the viewfinder. Whatever is inside the sensitive area, the AF will often go to the bit with most contrast that it can lock on to most easily, or it may choose the nearest part.