It's called Litotes
In rhetoric, litotes (/ˈlaɪtətiːz/,1 US /ˈlɪtətiːz/ or /laɪˈtoʊtiːz/) is a figure of speech wherein understatement is used to emphasize a point by stating a negative to further affirm a positive, often incorporating double negatives for effect
For example, "She's not bad looking" could be used to express that someone is gorgeous. Or it could convey that she's not particularly ugly, but also isn't particularly attractive.
From Oxford:
Ironical understatement in which an affirmative is expressed by the negative of its contrary (e.g. I shan’t be sorry for I shall be glad).