What is is.
The number of bytes transfered in response to the request.
What it says.
It tells you about the traffic that has been caused by the request.
This possibly is the only "honest" information in the whole log entry. While other statistics might be skewed by proxies and caches, this really is "what went over the line". This information is important if you're paying by volume, or have a certain quota you don't want to exceed.
A look at the operation tells you the direction of the transfer (either from or to the server).
What may cause trouble.
Nothing I'm aware of. Do you know something?