A horizontal brace placed above expressions to group terms or indicate scope, often with an optional annotation above.
Examples
Grouping terms with explanation above
\overbrace{x + y + z}^{\text{sum of terms}}
Marking sequence length
\overbrace{a_1, a_2, \ldots, a_n}^{n \text{ elements}}
Highlighting part of an expression
f(x) = \overbrace{(x-1)(x+2)}^{\text{quadratic factor}} + 5