In general, what makes it properly basic is that it is justified. The word "properly," in this context, means "justified." A properly basic belief is a belief that has two properties: (1) it is justified and (2) it is not justified by inferences from more basic propositional beliefs (a "propositional belief" is a belief whose content is a proposition rather than some other kind of mental representation like a visual image, etc.).
Typically properly basic beliefs are beliefs that are justified by experience. Suppose, for example, that I am in a classroom and see an apple on the teacher's desk in front of me. Suppose also that I form the belief that there is an apple on the desk in front of me and that belief is justified. In this case, my belief is not justified by other propositional beliefs but directly from the sense experience itself.
One way to think about this example might be like this: the mental image of the apple on the table is doing the work of justifying my belief but notice that the image is not a propositional belief. It is just an mental image, or representation, we experience in "our mind's eye." The experiencing of the mental image is what supposedly justifies the belief, not an inference, or argument, from other more basic propositional beliefs. So, properly basic belief are beliefs that are justified but are not justified inferentially on the basis of other propositional beliefs but in some other, non-propositional, way.
More specific to Plantinga's later work, a belief is properly basic if it is formed in the basic way and is warranted. Roughly, warrant is like justification. According to Plantinga, roughly again, a belief is warranted if it is produced by cognitive faculties designed to produce true beliefs given certain kinds of inputs in particular cognitive environments. So, for example, beliefs produced by our visual faculty in an environment with good lighting and looking at an objects near to us, are properly basic because they are warranted but not warranted inferentially from more basic propositional beliefs. Rather they are warranted by our visual faculty producing a belief about an apple on the desk and it was designed to do so in that kind of environment. (Presumably it is the mental image of the apple on a desk that is caused by our visual faculty which produces true beliefs in that environment). Other cognitive faculties involve memory, reason, etc.
