what are the blood types, what blood type is the most common in most humans, and what do the positive and negative signs mean?

http://www.redcrossblood.org/learn-about-blood/blood-types

Blood types are classified based on the presence or absence of certain antigens on the surface of red blood cells. There are four main blood types: A, B, AB, and O.

To determine the most common blood type, you can refer to statistical data. According to studies, blood type O is the most common in most human populations globally. Approximately 45% to 50% of people have blood type O.

Regarding the positive and negative signs in blood types (e.g., A+, B-, AB+), they indicate the presence or absence of another antigen, called the Rh factor. If you have the Rh antigen on your red blood cells, your blood type is Rh-positive (+), while its absence results in Rh-negative (-). The Rh factor is another important consideration when determining blood compatibility for transfusions or pregnancies.