C3 and C4 refer to the number of carbon molecules in the sugar that the plants create. I took biology 1 at a community college a few years ago and we talked about this.

C3 plants are under threat by climate change while C4 plants are able to adapt to a warming planet. Which is scary because most of our crops are C3, like wheat, rice, and barley. Corn is C4 so we might be eating a lot more corn in the future.