Meaning: A person who works on a farm does something that requires effort or skill with difficulty or requiring a lot of effort the whole amount of something from sunrise to sunset.
Choice of Words: A "farmer" is a person who works on a farm. "Works" means to do something that requires effort or skill. "Hard" means difficult or requiring a lot of effort. "All" means the whole amount of something. "Day" is the period of time from sunrise to sunset.