I've been looking for a concise explanation for this question for a while. What I found were confusing and not very didactic texts.
As far as I know there is no clear and universally accepted definition of what this term means.
Others might say that they are classes with a very short size and that they can be copied more easily rather than being passed in as a reference. OR it could still be one that doesn't have a virtualization table, although less likely.
In certain contexts there may be a specific explanation.