The subtitle, simply put, is a metaphorical statement that contextualizes the overall title within the historical events Spiegelman's graphic novel describes. To some extent, it could also be considered a form of hyperbole, but given the scale of bloodshed in the Holocaust, it is not really an exaggeration at all.
A metaphor is a literary device in which an idea is expressed figuratively rather than literally and in which a comparison is implied between seemingly unlike things. No one can literally "bleed history," of course. But the idea conveyed by Spiegelman is that the immense suffering of the Holocaust is like that of blood being poured out, and the essence of it is analogous to the historical process. The Holocaust is the focus of Maus, but "history" as a whole is an endless series of wars, mass murders, and atrocities. These are symbolized in the metaphorical reference to one's father's "bleeding."
The literary device on which Maus in its entirety is based is anthropomorphism, the symbolic attribution of human characteristics to non-humans. The metaphor of the complete title of the book is one which combines the anthropomorphic element (using the German spelling of the word mouse) with the juxtaposition of the concrete act of bleeding and the abstract concept of "history."
The subtitle of Maus is a metaphor. "My Father Bleeds History" is a clever, symbolic expression that could stand for several different allusions.
Merriam-Webster defines metaphor as:
a figure of speech in which a word or a phrase literally denoting one kind of object or idea is used to in place of another to suggest a likeness or analogy between them
Art Spiegelman's use of this literary device can be interpreted in two ways, I believe. One analogy refers to the sensation of pain, brought to mind by the purposeful use of "bleed" in the subtitle. The author's father does not "exude" history, nor does he "radiate" or anything of that kind. He bleeds. It is much more fitting to the subject matter, the Holocaust, and the pain that brought: not just living through the horror in the first place, but being persuaded to speak of it.
The other possible meaning I would offer has to do with the word "history." It would be hard to argue against the fact that some people have gone through more hardships in their lives than others. Art Spiegelman's father is certainly one of them, a survivor of one of the greatest crimes committed against humanity. Saying that he "bleeds history" is to say that in a way, he has become history. The very blood in his veins has become inseparably mixed with the trauma - it's not so much a matter of just bleeding, but Vladek can't help but bleed history. You could compare it to certain locations around the world where something so powerful has happened that visitors profess to "feeling" some remnant of the events, as if the air carries some memory of it. And so it is with Vladek as well. Around him, history is just... closer, somehow; and more real than if it was mere fiction.
In the subtitle to the first part of Maus, the phrase "My Father Bleeds History" includes a metaphor. In this part of the biography, Art Speigelman gives the background to his father's, Vladek's, story. Through interviews, Vladek tells Speigelman the details of his life before he was taken into concentration camps. Speigelman uses the word "bleeds" as a metaphor to describe the painful details that Vladek reveals to him. Plus, Vladek who has not been so open with his son throughout his life, finally opens up to Speigelman and gives him many details. So, the use of metaphor in the subtitle of the story suggests the nature of the story that Vladek shares with his son.