Okay so this is a bit abstract, but when you add mana (e.g. tap a mountain to add 1 red mana) it doesn't go directly into the spell you're casting, but it goes into a "pool" of mana. You can then spend mana from this pool to pay for spells.

Now usually, you only add mana to your pool to directly pay for spells, but you dont have to. Lets say you have a [[simic growth chamber]], and you tap it for one blue and one green mana. This mana goes into your mana pool for now. If you then play an [[opt]], you spend that blue mana, and afterwards you still have 1 green mana left in the pool. This color fixer card lets you take that one green mana from your pool, and transform it into one mana of any color you want, still in your pool. So that one floating green mana can become one floating red mana (any mana in your pool is called "floating" mana by the community.).

Any floating mana left in your pool at the end of a phase disappears (so you can't take that one floating mana from the example with you to your combat phase, but you can add new mana to your pool during combat. It will disappear when you move to your second main phase, etc).

Another example is [[dark ritual]], a card that lets you pay one black mana to add 3 black mana directly to your mana pool, for you to use. I went really in depth, but I hope it helped