The function std::transform in C++ is used to apply a specified operation to each element in a range, transforming the input data into a new output. It takes three main parameters: the beginning and end of the input range, and an output iterator where the results will be stored. An optional fourth parameter can specify a custom operation, allowing for flexibility in how the transformation is performed. This function is part of the algorithm library and is commonly used with containers like std::vector or std::array. It simplifies code by eliminating the need for explicit loops, making it easier to read and maintain while enhancing performance through optimized implementations.