Light will pass through a pair of Polaroids when the axes are aligned. Because each polaroid can only pass light waves in one direction through it - either horizontal or vertical. If the axes are aligned at right angles to each other, then the horizontal polaroid will not allow the vertical light waves to pass, while the vertical polaroid will not allow the horizontal light waves to pass. Therefore light cannot pass through a pair of polaroids when the axes are at right angles to each other.
