As far as I know, it's an incantation, either of Hebrew or Roman origins. So I went to wikipedia and found out that the Hebrew explaination makes the most sense. It's an amulet you wear around your neck for 9 days that looks like this:
Supposedly, this diminishes the hold over the patient of the spirit of the disease, or demonic possession.(I can't get the characters to make an upside-down pyramid, the arrow keys don't work on it properly which is creepy)
I'm going to copy and paste what I found on the wikpedia article to better explain where the letters might come from.
the source is Hebrew and the ABRACADABRA has to be read from right to left as in Hebrew. ABRACADABRA, when phonetically pronounced from right to left, reads in Hebrew ארבע-דאח-ארבע. The Hebrew word דאח is the word אחד with rearrangement of the letters. This was done to confuse the daemon or for various witchery reasons. The evil, dark forces and the daemons' kingdom in the Jewish Kabbalah are represented by the number 9 (the Hebrew word תשעה) or the Hebrew letter ("טית "ט). Since the ABRACADABRA word has to diminish a letter a day for nine (9) days it has to have enough letters. The Hebrew word תשעה is too short because it has only four letters. Therefore, the Hebrew word ארבע-אחד-ארבע was created. The Hebrew word ארבע is the Hebrew word for the number 4 (four) and the Hebrew word אחד is the Hebrew word for the number 1 (one). Therefore, ארבע+אחד+ארבע equal 4+1+4=9 and it represents the dark forces and daemons' kingdom. This explanation for ABRACADABRA succeeds where all other previous known explanations and sources did not explain why the patient has to wear the amulet for nine (9) days. In Hebrew the "ABRACADABRA" should read as "ארבעאחדארבע".