Adjusting the ADC prescaller

I know there is a way of adjusting the ADC prescaller using cbi and sbi but I don't know how they work. Any clues would be awesome.

Using google I found some snippets:)
http://www.google.com/search?q=arduino+ADC+prescaler+cbi+sbi
One of them:

// set adc prescaler to 32 for 38 kHz sampling frequency
sbi(ADCSRA, ADPS2);
cbi(ADCSRA, ADPS1);
sbi(ADCSRA, ADPS0);