I was looking though some old music today and I found a symbol I did not recognize. This symbol as shown below looks like a square bracket next to two notes.
So what is the bracket suppose to mean?
It's an indication that you are supposed to play the bracketed notes with a single finger. In this case—as is most common—you are supposed to play both notes of the second with your thumb instead of two fingers.
At a guess (given that the following note is an octave down and probably played with 5), play the two notes with the thumb, Dom, i.e., with thumb on the crack between the keys.