I've written a bit about the topic of literacy in Ancient Rome before here which might be of interest. In short these graffiti are an indication of some level of widespread literacy, and in recent times, excavation techniques more sensitive to small details tend to produce more results of graffiti from around the empire rather than just Pompeji. But your question hits it right on the head: 'read and write, at least a little' would be the extent of most 'literate' Romans literacy. Literacy is a very grey area on the spectrum between not knowing what to do with those squiggles and writing poetry in perfect metre, and it's hard (or rather impossible) to get good numbers on how many and how good. Certainly more than the pessimistic estimates, certainly less than people today.