Understatement

What is an understatement?

An understatement is a figure of speech used by writers or speakers to intentionally make a situation seem less important than it really is. This can be done for an ironic effect or to be humble and polite.

After winning a bumper prise an understatement would be I am kind of happy 

Another example 

A guest at your table breaks a plate and you say it’s hardly noticeable.

Your Expensive car bumps into a tree and you compensate the loss by saying It’s a small loss.

We can also use understatement to create humour. They lower down the severity of a situation and draw the attention of a reader.