This notation indicates a harmonic. Harmonics are played by lightly touching a string at one of several specific points along its length, preventing its fundamental tone from sounding while allowing certain overtones to vibrate. Which overtones vibrate, and hence what pitch is produced, depend on the fractional point, referred to as a node, at which the string is touched.
There's a distinction between natural and artificial harmonics. Natural harmonics are played on an open string. Artificial harmonics are played on a stopped string, typically stopping with the first finger and touching a node with the fourth finger.
There isn't a standard way to distinguish between natural and artificial harmonics in notation. Some harmonics can only be achieved by stopping the string, while others, such as the one in the question, could be played either way. The example could be played as a natural harmonic, by touching the open D half way along its length, or as an artificial harmonic, by playing in fourth position on the G string. Where there's a choice, it must be made based on the musical context.
Without more context than the single bar of music posted, it's impossible to say which choice is correct here. What's coming next? If the next change of note is, for example to a harmonic one tone higher, maybe it makes more sense to play the D in fourth position then shift up. Maybe there's a glissando coming. Maybe vibrato is appropriate, in which case an artificial harmonic is needed, since you can't vibrate a natural one.