It's similar to the kama sutra in that it's a set of sex positions. I'm pretty sure the actual positions have varied over time, but the number 48 has been around for a few hundred years, likely because it's considered a lucky number in Japan and other places in Asia. I can't name any off the top of my head, but I know the 48 positions have been referenced at least once or twice in chapters I've translated here on Fakku.
Ah yes, another one of the 48 sex positions.
'A popular choice on cold nights', eh? Well, a kotatsu warms you up, and sex also warms you up...So double the effect?