In front of Huayna Picchu is a massive slab of rock that is said to take the shape of the mountains behind it. This rock, like all large rocks, was sacred to the Incas.
A major part of Inca beliefs were that such rocks store vast sums of energy that can be transferred to people. All things in nature were similarly thought to be special in some way, from water to the Sun.
Of course, the biggest rocks that exist are the Andes themselves, and this is why the Incas built everything at the tops of them, the pinnacles of the stores of energy.