The bottom left temperature is the inside temperature of the passenger cabin, the right hand side temperature is that of the outside air (as hinted at by the sun and cloud icon).
For input, there will be three push buttons (or capacitive touch buttons, I haven't decided yet). They will serve as "back", "set" and "forward" controls.
I agree that icons have to be mostly self-explanatory. But personally I think I could have done worse in designing these icons. You should be able to figure out most of my icons within three to five seconds of seeing them for the first time.
"rem" stands for "remain", which in this case will be the remaining fuel in the tank in liters (there might be a "mile and gallon" mode where you will get an MPG icon instead of l/100. I haven't decided yet if I want to go to that length).
If this trip computer really ever goes into mini-production beyond the specimen that I am building for my own car, then there will almost certainly also be a leaflet explaining all the trip computer's functions and features from beginning to end. Modern-day cars with all their interactive touchscreen gadgetry also tend to come with quite sizeable manuals explaining in detail every single icon... 