Jeez is a short spoken word people use when they are surprised, angry, annoyed, or a bit shocked. It is like “oh no” or “for goodness’ sake,” but rougher. You might say “Jeez, it’s cold” or “Jeez, would you stop that?” It is very informal. You will hear it in speech and in chat messages; you would not put it in a school essay or a polite email. Though actually, it is a softened way of saying the name Jesus as a cry, without quite saying the full name. Some religious people still find it rude, because it treats a holy name as a sound of annoyance. You may also see geez, same word, different spelling. It is not a verb and not a thing — it is just an exclamation.