weaken or soften a statement
A statement is deliberately weakened to sound ironical or softened to sound more polite.
Note that understatement is a common feature of the English language (especially British English) used in everyday-life situations.
- I know a little about running a company. (a successful businessman might modestly say.)
- I think we have slightly different opinions on this topic. (instead of: I don't agree with you at all.)
counterpart: → Hyperbole