Blood type refers to the type of cell marker that exists on the outside of your red blood cells. There are 3 types- A, B, and O. If you have A, B is considered foreign so you will have anti-B antibodies. The same can be said for A if you have B type blood. If you have O, it is neither A nor B so you make antibodies for both A and B. AB blood types have no antibodies since both types are present on their cells. Type ABs are referred to as "universal recipients" because they have no antibodies in their blood and can therefore receive any type of blood without fear of the body rejecting it. Type Os are the universal donors under most condition but this can vary depending on other more complicated factors since as I mentioned before they have anti-A and anti-B antibodies.
These types are genetic. A and B are dominant to O, and if you get an A from one parent and a B from another, you will be AB (co-dominant).
Complicated topics explained, simply.