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The phenomenon of our blood group being positive or negative is the rhesus factor(or rh factor).
The rhesus monkey was one of our genetic ancestors.
1.Those who have rhesus genes are have positive blood groups(rh+)
2.Those who don't have rhesus genes have negative blood groups(rh-)

Most people have positive blood groups(about 80%)
So if you are positive then you have more chances of getting blood when needed and if you have negative you have a slightly less chance.

blood group is determined by the Rh factor in our blood. there are majorly A positive, A negative, B positive, B negative, AB positive, AB negative, O positive and O negative.
in the world most of the people have the positive groups of blood. but only few have the negative group of blood.
it is nothing bad if we have negative group of blood. but we would be having a disadvantage that we cannot get the same group of blood. but in the positive group of blood we can get the blood easily.
the people who has positive blood group know they can get the blood sample easily. but it is hard for the people to get negative blood.