The cell quality metric, or more fully, the cell quality gradient metric, describes the quality of the cell based on an estimate of the finite volume approximation of the gradient reconstruction at the cell centroid. It can be thought of as a measure of the distribution of cell centers around a cell in terms of accessing the gradient.
A cell with a quality of 1.0 is considered perfect. As the cell quality approaches zero, the cell becomes more degenerate. Depending on the physics selected for the analysis, the cell quality of a cell can be fairly low (of the order 1e-6 or less) and still provide a valid solution.