The integral symbol is used to represent the integral operator in calculus. Typically, the integral symbol used in an expression like the one below.
In plain langauge, this means take the integral of the function with respect to the variable from to . See integral notation for typesetting and more.
An integral can be geometrically interpretted as the area under the curve of a function between the two points a and b. The concept is used throughout physics and higher level mathematics.