A big reason for a toddler having a tantrum is to release frustration over something that is causing them to be upset.
As a parent you do not particularly want to reward the toddler tantrum behaviour because this could lead to more tantrums in future.
What do you call REWARD? When a toddler is in the midst of their tantrum the parent can do two things:

1. Give the toddler lots of hugging and caring and try to appease the tantrum - this can work and is a direct mechanism of supporting the poor behaviour - toddler view is throw a tantrum and get lots of love
2. Give the toddler discipline - telling a toddler off and giving them a reprimand is another approach.
   Surprisingly this can also be the wrong thing to do as it re-enforces poor behaviour in the toddler because again they get attention and any attention is good to some toddlers.
   

So what as a good parent are we supposed to do? The idea is to react in a way that is neutral but keeps the toddler safe without strong attention one way or the other.
In a situation where the toddler is in the throws of the tantrum and is possibly shouting, screaming and red-faced a neutral response would be to simply hold the toddler and let him/her know you are there to care for them.
This approach is keeping them safe (from falling over, hurting themselves) without giving them the positive (loving) or the negative (discipline) attention queues to trigger the tantrum behaviour next time.
When the tantrum has blown itself out the toddler will know that you are there to keep them safe as a parent but also that the behaviour itself does not bring any particular reward one way or the other.