Fold expressions are a feature introduced in C++17 that allows you to perform a fold,
or an accumulative operation, over a parameter pack. They are used to simplify code
when you have a parameter pack that you need to perform an operation on, such as
summing or concatenating elements.

Here's a simple example of using a fold expression to sum a variadic list of integers:

template <typename ... Args>
auto sum(Args ... args)
{
    return (... + args);
}

int main()
{
    auto result = sum(1, 2, 3, 4, 5);
    std::cout << result << std::endl; // Output: 15
    return 0;
}
In this example, the fold expression (... + args) performs the binary addition
operation + on each element in the args parameter pack. The operator ... spreads
the elements of the pack and the binary operator + performs the addition. 
The result of this expression is the sum of all the elements in the pack.