Some things i'm trying to understand on Kevin's Accelerometer Code

[Squall]

We're trying to get our accelerometer to work here with Kevin's Code, that part is easy, but we're trying to understand everything on the code so that we can make the modifications we need.

We're having a special interest on this part :

adc_result = ADRESH;
adc_result << = 8;
adc_result += ADRESL;

We've understood that it's reading the High bits of the ADC and then it'll copy it do the lower part of the adc_result variable, this part I really can't understand. Why does he do this ?? I've been trying to figure out for a while now, does anyone have a clue ?

Thanks