It's called litotes.
From Wikipedia:
In rhetoric, litotes 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.
The degree of emphasis depends on the context in which it is used.
From Oxford:
litotes: [MASS NOUN] 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).
