What electronics should I study to create something like a fridge temperature monitor?
A fridge temperature monitor (or any system that talks to world using some data) will include a sensor (temperature sensor like LM35), a controller (to do calculations, arduino ofcourse) and an output (display, like LCD). The topics you should be aware of, are digital electronics, basic passive components (working of resistor, capacitor etc). And a little arduino programming and lots of logic.