What is confusing you about the ability?

So the "currency" in the game is mana. Mana is and gray circles with numbers. Most lands produce mana.

Normally, any unused mana in your mana pool goes away when a phase or step ends (When you move onto the next part of your turn). This card keeps green mana (:g from emptying from your mana pool.

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So let's say you have three forest s. And you draw a Grizzly bears . You can tap all three forests for green mana. You can spend two mana to cast the grizzly bears and that last mana will sit around forever until used (or until Locus leaves the battlefeild).

So next turn when you draw a 4 mana card you can tap your three forests, and use your saved mana to cast it.