The std::sample algorithm is a new algorithm, which came with C++17. Its signature looks like this:

template<class InIterator, class OutIterator,
         class Distance, class UniformRandomBitGenerator>
OutIterator sample(InIterator first, InIterator last,
                   SampleIterator out, Distance n, 
                   UniformRandomBitGenerator&& g);

The input range is denoted by the first and last iterators, while out is the output operator. These iterators have exactly the same function as in std::copy; items are copied from one range to the other. The std::sample algorithm is special in the regard that it will copy only a part of the input range because it samples only n items. It uses uniform distribution internally, so every data point in the source range gets chosen with the same probability.