Question about losing data if I turn off iCloud.

I tried to explore what iCloud is and what it is for. I was asked to sign in, which I did. I don't know what I did next but I found myself on a screen in which different parts of my computer were being uploaded to iCloud. I have a McBook Pro running OSX 10.7.5., 2.3 GHz Intel Core i5, 4 GB 1333 MHz DDR3 Capacity is 319.21 G, available 115.4GB. There were a checkmark on every itemexcept mail. I wanted to stop the uploading so I started to uncheck things. When I unchecked Documents & Data, I got a message saying "If you turn off Documents &Data, all documents stored in iCloud will be removed from this Mac. Your documents will still be available on other devices using iCloud." Then I was given two choices: "Cancel" or "Delete from Mac." But I don't want to delete anything from my Mac. I want to delete it from iCloud. There is a bar along the bottom of the window, showing the progress of the upload. The portion of the bar in green appears to be 5% or less of the full bar. Above the bar it says "iCloud Storage: 4.83 GB of 5 GB storage. I don't know what that means. Have I already used 4.83 GB of storage? Am I going to lose some documents and data if I choose Delete from Mac? And why should that be so? Is it not reversible.? If so, this is terrible. The iCloud window is still open with the message window on top of it. I am leaving my computer on and going to bed. I hope someone will be able to answer me by morning and tell me what to do.. Please help! Thank you