The pH scale is a simple numerical scale from 1 to 14. If you take a piece of Litmus paper and dip it in a solution (we'll go with rain water if you like) and it comes out neutral, the pH value would be 7, right in the middle. It would be neither acidic, nor alkaline, and that's how rainwater is supposed to be...pure. If pH is below 7, the water is acidic and if above 7 it is alkaline. Thus, values of 6 and 8 would be slightly acidic and slightly alkaline, respectively. A pH value of 1 or 14 would be an extremely dangerous substance (either very, very acidic or very very alkaline).
The pH of modern rainwater is between 5.5 and 6.0 because of carbon dioxide that's dissolved in the atmosphere. This CO2 reacts with the H2O (water vapor) in the atmosphere for the following reaction:
CO2 + H2O ---> H2CO3 (carbonic acid)
then that dissociates:
H2CO3 ---> H+ + HCO3- (bicarbonate ion)
the bicarbonate ion can then dissociate:
HCO3- ---> H+ + CO3- (carbonate).
All of these reactions are reversible as well and is what makes bicarbonate a really good buffer. Bicarbonate is the same buffer used in human blood.