A camera lens opens for only a fraction of a second, say 1/250th of a second.
But if it is dim or dark (and you dont use a flash) then the lens can open for longer, maybe 1/50th of a second.
That extra time, even if it is only a fraction of a second, can be enough to give camera shake (your hand shaking).
The other problem could be that you took the picture before the camera had time to focus.
Many cameras have a two stage approach for taking pictures. You hold the button down a little bit to prepare the camera (focus it for example) THEN press the button to take the picture.
If you were taking lots of pictures very quiuckly the camera may not have had time to get ready.
You may be able to sharpen it with some software products.