Dear George,

There are Mitzvos that must be performed like Bris Milah, Kidush on Shabbos, Krias Shma twice a day- each is Mitzva Chiyuvis, obligated.

This is opposed to doing Chesed, wearing Tzizis, slaughtering properly, etc. that is called Kiyumis (done when carried out). Obviously we are commanded to do them, but compared to Chiyuvis it is more "voluntary".

All the best, Reuven Weiner