Let $I$ be a 3-by-3 identity matrix and $\hat n$ be a unit vector orthonormal to some surface. What then does $I-\hat n\hat n$ mean geometrically? Also what does it mean to multiply this matrix/operator by a vector $v$? Some sort of projection?
Also, I don't understand what $\hat n\hat n$ means. Dimensionally it should be a 3-by-3 matrix because $I$ is such, right?