Imagine a plane perpendicular to the double bond that cuts the double bond in half. That plane also runs through the carbon atom represented as the vertex at the top of the structure as drawn and runs halfway through the single bond on the other side of the molecule. That's the plane of symmetry for the molecule, there is a half coming out of the computer screen and an identical half going into the computer screen, and the presence of that plane means the molecule is achiral. There are two carbon atoms with four different substituents, but those two stereocenters are part of the larger system of symmetry, so there would be a single meso compound.
If there were a double bond on both sides then there would be two planes of symmetry. The second plane would divide the molecule into a left and right half.