Thanks IKEA

As someone who's home is 95% IKEA, I'm surprised this needs to be pointed out. What they mean is that you screw n the screw but not all the way tight, to leave some wiggle room when connecting the other pieces. Then screw them in, but not tight. Last, when the whole thing is put together, you tighten all the screws. Take care not too over tighten since this will break the wood or plastic, and come back a month later and retighten everything, since the wood has moved and settled a bit. This will make even the cheapest IKEA shelf stay good for decades.