Magic: The Gathering: Mana?

Quite a bit of a noob here, just getting into MTG If I add mana to my mana pool by an ability [For instance, Rofellos, Llanowar Emissary] do I have to use mana cards from my deck? Let's say I have 5 Forests on my field and In play Rofellos, Llanowar Emissary, which I can tap to add 1 Green mana for each... show more Quite a bit of a noob here, just getting into MTG
If I add mana to my mana pool by an ability [For instance, Rofellos, Llanowar Emissary] do I have to use mana cards from my deck?
Let's say I have 5 Forests on my field and In play Rofellos, Llanowar Emissary, which I can tap to add 1 Green mana for each Forest I control, do I have to get 5 forests from my deck or is it just sort of there, meaning I have to keep track of it somehow?
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