1. expressing with restraint or lack of emphasis, usually used for an ironic' or rhetorical
2.basically a figure of speech that writers use to subtly make a situation less important or serious than it really is.
Saying "We've had a little rain," when the neighborhood is flooded. Now that is an understatement.
The last line of Frost's "Birches" illustrates understatement: "One could do worse than be a swinger of birches."