Understanding the algorithm of Visual C++'s rand() function

In C/C++, rand() and srand() are usually used by us when we want to get a random integer. But when I tried to rewrite it myself, I found it difficult to understand the algorithm. The function is very easily written in only a few lines, but the formula is misunderstanding.

The main formula:

ptd->_holdrand = ptd->_holdrand * 214013L + 2531011L;

The original code involved:

void __cdecl srand (unsigned int seed)
{
    _getptd()->_holdrand = (unsigned long)seed;
}

int __cdecl rand (void)
{
    _ptiddata ptd = _getptd();
    return ( ((ptd->_holdrand = ptd->_holdrand * 214013L + 2531011L) >> 16) & 0x7fff );
}