Ok, let's sort this out. Dock items are like shortcuts in Windows. When you lose one, it's not a tragedy. We can fix that easily.
Folders in a Finder Window are the real deal. They correspond to directories in Windows, and hold the files, programs, etc. that you need to use your Mac.
The OS will not allow you to put your actual Documents folder in the Trash. An error message will show up and refuse to proceed with the action. You also cannot move your Documents folder. The only action the OS will permit is a copy.
To be reassured that you still have your Documents folder, double click your Hard Drive Icon, then your User folder (the one with the house) and it should be there.
Back to the Dock now... Drag that Documents folder to the right end of your Dock (or the bottom if the Dock is on the side). Release, and you should now have it back. Do the same for the Downloads folder that should be right next to the first one.
You're not all set just yet. For some reason, it will be displayed now as a Stack instead of a Folder. This is a bit confusing, but it's not my choice of terms. Right click (or control-click) on one of the two newly created Dock icons and look under Display as and pick Folder, because Stack will be selected. (even though Folder was the default when the computer shipped) Now you should be all set.
The confusing part:
When you drag a Folder icon to the Dock it creates a Stack. The definition of a Stack is a shortcut to a folder that is in the Dock. Why the Display as choice is between Folder and Stack is beyond me. Maybe it should say Display dock icon as and give you the choice of file or folder. That's the effect it has.