The function std::count_if is part of the C++ Standard Library, specifically within the algorithm header. It is used to count the number of elements in a range that satisfy a specific condition defined by a predicate function. This function takes three parameters: the beginning and end of the range, and the predicate itself, which returns a boolean value. When you call std::count_if, it iterates through the specified range, applying the predicate to each element. If the predicate returns true for an element, the count is incremented. This makes std::count_if a powerful tool for filtering and counting elements based on custom criteria in collections like arrays or vectors.