What is the different between high resolution and law resolution

I can understand there are quality differences. but I am a beginner to web development and i saw every theme and plugin have a detail option about resolution.i want to know
1.is it helping to speedup the web site lording ?
2.can a law resolution script convert to a high resolution.i mean with customization ?
The real reson i am asking this,i am going to give someone to make a web site for me,so i want to know about everything before the first meeting . so plz guys if you can explain for me this will be very helpful for me.