This is the accompaniment piece for a clarinet solo and I do not know how to play this measure.
It looks like a tremolo. It is played as a oscillation between the top two notes (I'm guessing a A and a D) and the lower note (I'm guessing an F). The dotted half-note (written on each stem) indicates either 3 quarter-note beats or 6 eighth-note beats (or the equivalent); the 6 eighth notes in the other staff probably mean the latter. The three lines connecting the upper notes mean to (approximately) play the oscillation as thirty-second notes. On a keyboard (which this looks like it's from) this gives a shimmering effect to the chords. It's a bit like a trill but with larger intervals and more notes.
It's a tremolo, which for a pianist means to alternate very rapidly between the two notes/chords. Here's a video that shows you how to do it.