How do you feel when you are in an unsettled mood and you don’t understand what is going on with you?
I would say it adds a layer of frustration and exasperation to an already unpleasant state, doesn’t it?
Your child probably feels the same. Often, they are still learning to understand their own emotions, and anger is a very unpleasant one.
So, these are some of the things that you may be able to share with your child:
1. Anger is an emotion.
It’s difficult to explain what an emotion is because we cannot see them.
Emotion is how we feel in our mind, our thoughts, moods and how it all makes our body feel. It is like a mental sequence in which: 
1) There is a situation or something happens(stimuli/relevant triggers)
2) Our brain decides if that situation is good, bad, dangerous, etc. (a cognitive dimension / we assess those stimuli) 
3) Our body reacts to that situation. Our heart may start beating fast, we may sweat- all our body sends us signals that may be different depending on the emotion we are feeling (physiological arousal) 
4) We feel we need to express how we are feeling (behavioural dimension)
So, when it comes to getting angry, this example could explain how this sequence works:
1) My sister breaks my favourite toy
2) This is bad news! I think it’s a disaster, I love my toy, and she was not supposed to take it. 
3) My ears feel hot, my heart races, I’m so mad I can’t talk
4) I shout at her (hopefully nothing worse!)
When I feel angry I may feel irritable, tense, and anxious. I may also have negative thoughts. And my body starts sending me “anger signs”.
2. BUT, anger is not bad.
All emotions are there because they fulfill a purpose.
3. Anger’s “duty” is to defend yourself.
Your body is telling you that something is bothering you (or threatening you). And you will be able to decide what you need to do to solve that situation
4. We all feel angry sometimes
Grown ups also feel angry sometimes.
Share an example with you child about a situation when you felt angry, and how you dealt with it in an appropriate manner. 
5. Good news!
We can learn ways that help us control our anger. 
Sometimes we will need to solve a problem. 
Some other times we will not be able to fix what is bothering us, or we will not know exactly what is causing these feelings. In those cases, we can still learn ways that help us control our anger