{T}: Add {C}.
{T}, Pay 1 life: Add {U}.
{1}{U}, {T}, Sacrifice a Desert: Target player mills four cards.
If a Desert has an ability with a cost of “Sacrifice a Desert,” you can sacrifice that Desert to pay the cost for its own ability.
Desert is a land subtype with no special meaning. It doesn’t grant the land an intrinsic mana ability. Other cards may care about which lands are Deserts.