Ich sehe ihn ab und zu.
Duolingo says this means "I see him once in a while." I'm not really sure of the grammer behind this since it looks like "I see him away and to." or something odd like that.
As the others said - it's just a fixed idiomatic expression meaning "once in a while", "every now and then/again", "from time to time", "off and on", "occasionally" etc. There's a cute song by the German Cookie Monster (Krümelmonster) called "Kekse isst man nur ab und zu" (You [should] eat cookies only once in a while). Maybe listening to this song will help you remember this expression :) http://www.youtube.com/watch?v=hPwbfolmNMM