Photosynthesis

What is photosynthesis?
(Apart from a long word)
It's really quite important,
For all the plants I've heard.

It's something about breathing,
And carbon dioxide,
But I don't really understand,
No matter how I've tried.

It's something about water
And sun and dark and leaves,
And something about oxygen
Which every plant it breathes.

In school my teacher told me,
His name was Mr Rotten,
It's something about glucose,
But now I've quite forgotten.

So can you tell me anything
About a word as long as this
I'd really like to know, you know
What's photosynthesis?