Miter boxes (rod-shaped in some parts of the world) are devices that help guide a saw so that it can cut an exact angle, such as 45 degrees. Lots of commercial options are available, usually made of plastic, but you can also make your own miter boxes with just a few pieces of wood.
The most typical design is a bottom piece with two sides that extends upwards and creates a channel for the wood that you cut to fit in the middle. Using an angle search tool, you can then draw which angles you want to be able to cut routinely (45 and 90 degrees are popular choices) and then cut them through the sides. When you need to cut a certain angle, just push the material you are cutting into the channel, place the saw in the corresponding line and cut away.