The mirrors in a reflector are not attached to the telescope like the lenses of a refractor. They can and will move around slightly over time. This results in blurry images. Having the mirrors perfectly aligned is therefore of the upmost importance. Collimation is the process of aligning the telescope mirrors. Unlike refractors, reflectors need to be collimated once in a while to keep and this adds a little to your setup time.

I simply collimate every night just to be sure.

Cheshire Eyepiece

This can be done using a cheshire. Simple, reliable, no batteries needed but you can only use it during the day.

Laser Collimator

Call me lazy, but I chose not to use an analog cheshire but a battery-powered laser collimator. It allows me to collimate when I am setting up my telescope, which is often when it is getting dark or pitchblack already.

Here is a nice video describing how to use them.

Now if you use a laser collimator to align your telescope mirrors, make sure the laser collimator is collimated itself.