Sub genre is a genre that is part of a larger genre or main genre.
It's a sub division of a genre.
For example I want to write a romance novel. It's not only about how he or she has feelings. I have to add other things. Those things are the sub genre.
The main genre is romance but I'd still add more genres.
For example I would write a romance story that takes place around the Devil and an Angel.
It's romance but it'd have hints of may be thriller, paranormal, mystery and more in the plot.
There are some romance stories that gruesome deaths are involved.
We also have Sci fi as the main genre then when maybe they're trying to defeat the cyborgs they start discovering their feelings.
Here romance is the sub plot and in the story there may be mystery or thriller.
If this topic wasn't explained well, PM me and I'd work on it.