Isometric graph paper
Three sets of lines at 30° to the horizontal, forming equilateral triangles. The standard grid for drawing three-dimensional objects without perspective distortion.
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Isometric is not perspective
On an isometric grid, parallel lines stay parallel and a metre at the front of the object is the same length as a metre at the back. That is exactly what you want for technical drawing, exploded diagrams and anything that needs to be measured off the page. It is exactly what you do not want if you are trying to draw how something looks to the eye — for that, use a perspective grid instead.
Reading the three axes
The vertical lines represent height. The two sets of 30° lines represent the two horizontal directions. A cube is drawn as three rhombuses meeting at a point, and because every edge follows a grid line, you can build complicated shapes by counting rather than measuring.
Uses beyond engineering
Isometric paper is heavily used for tabletop dungeon mapping, for planning pixel art and voxel builds, and for knitting and beadwork charts where the stitch geometry is triangular rather than square. If you want the same lattice without the visual noise of continuous lines, use isometric dots.
Printing it accurately
This only measures correctly if your printer doesn't rescale the page. In the print dialog set Scale: 100% — not "Fit to page" or "Shrink oversized pages". If you're not sure it worked, print the calibration page, measure the ruler on it with a real ruler, and it will tell you exactly what your printer is doing to the page.