You know what
12/17 means: you play the note at the 12th fret and slide to the 17th fret. In this case both notes are important.
But, when you see
/17, this means that the first note is really not important. You slide into the note at the 17th fret, but the starting note is not emphasized; this is just an ornament or part of the way that the line is phrased. In practice, usually you would start one or maybe two frets below the target note and just slide right on up to the 17th fret, without lingering at all on the first note; the idea is just to get that sound of sliding into the note to come out.
When you are trying to learn music from tab or even from staff notation, it is not a bad idea to listen to a recording to get help in understanding the phrasing and make sure that you aren't misinterpreting something. If you listened to the recording that this tab goes with, I bet that you could figure out what the notation was trying to tell you.