Wind-Scarred Crag "Add"?
What it means is that, when you tap Wind-Scarred Crag, you get one red or white mana (your choice). So it behaves like either a Mountain or a Plains[1], depending on what you need, but with the penalty that you can't use it immediately (because it comes into play tapped). You do not need to play any additional card, you just get the mana.

[1] Although any card that specifically references Mountain or Plains will not affect this card, because it isn't actually either of them.