A horizontal brace placed below an expression to group terms or provide annotations in equations and formulas.
Examples
Highlighting terms in an algebraic expression
(x+y)^2 = \underbrace{x^2 + 2xy + y^2}_{\text{expanded form}}
Labeling parts of a sequence
1,2,3,\underbrace{4,5,6}_{\text{middle terms}},7,8,9
Explaining components in a complex expression
\int_0^1 \underbrace{(x^2 + 2x)}_{f(x)} \underbrace{(3x + 1)}_{g(x)} dx