You're mixing 'land' and 'mana'. Your 'mana pool' is a count of how much mana you have generated from your resources. Imagine you tap a forest for mana - you now have 1 green mana in your 'mana pool'. Old versions of forests used to actually say that, but it's assumed everyone knows what Basic Land does, it taps for one unit of the pictured mana.
So lands generate mana, but so do other cards, like Birds of Paradise, and Dawn's Reflection. They are not implying you fetch land cards from your deck, they are just adding mana to that mana pool, like Basic Land does.
So imagine that forest is enchanted with Dawn's Reflection. When you tap the forest for mana, as well as that one green, you add two mana of any colour to your mana pool. That one land is effectively generating three mana, which you can then cast spells with.