That is in fact, the mana pool.
It is there to handle mana made by cards like
Gaea's Cradle or
Priest of Titania. (there are a few other lands and mana making creatures that also must use the mana pool)
These cards produce more than 1 mana and you cannot use them like regular mana sources that give 1 mana.
You must tap the cradle first, putting the mana into the mana pool. then when you play a spell or creature that uses green mana, you simply click the mana pool as many times as you need to pay for the spell or the mana pool is empty.
Example:
You have in play: 4 creatures, 1
Gaea's Cradle and 2
Forest.
You want to play
Sylvan Messenger.
First, tap the Cradle, putting 4 mana into the mana pool.
Next, click the Messenger and click the mana pool 4 times to pay for it.
But to pay for the Messenger, you could also use 2 mana from the mana pool and click the 2 forests, leaving 2 mana in the mana pool.
It is also possible to tap a land when not casting anything and thereby putting mana into the mana pool. But the mana pool was made to allow this game to handle cards that can produce more than 1 mana.
Just remember, just as in the real game, mana not used by the end of a phase is lost. If you go from Main Phase 1 into your Combat Phase, the mana pool is emptied and all mana still there is lost.
I hope this clears things up for you.