Someone who is fanatical is extremely devoted or insistent on something, often a religion or ideology. Sometimes this come up in political contexts as well, such as "a fanatical leftist/socialist/conservative." Borderline just means that the person is almost fanatical, so not quite there but close. The metaphor is that there is a border between fanatical and non-fanatical and the person is on that border "line."

first of all, "borderline" is a term used by psychologists that describes a person who is very close to having a mental illness of one kind or another. "fanatical" is an adjective that describes someone who is very excitable, and strongly opinionated about a certain topic. A borderline fanatical person might start an argument at a grocery store because their coupon for breakfast sausages said 3 packets for $3, not 2 packets for $3.99. Or a borderline fanatical person, would walk into a coffee shop and berate the employees for using styrofoam cups that some studies have shown cause cancer.