Really freaked me out when I saw something this uncommon in a beginner song. It's piano solo music if you need that info.
This indicates a lower mordent -- a decoration of the note that involves starting on the indicated note, rapidly toggling down to a lower auxiliary note, and then returning to the principal (written) note. The exact rate of the decoration is context dependent, but might be played like so:
(n.b. this specific interpretation was taken from the linked wiki page).
Note the vertical line through the symbol -- this is what indicates a lower mordent; the squiggle without the line would be an upper mordent.
Typically, the auxiliary note is a semi-tone below the principle note, though exceptions occur if the preceding/following note is itself a full tone below the principle, or if the principal note is the 3rd or the 7th of the (current) scale; c.f. this question, or this section of Grove's Dictionary of Music and Musicians.
It is a [lower] mordent, which is related to a trill. See https://en.wikipedia.org/wiki/File:Baroque_Trill_Instructions.png or https://en.wikipedia.org/wiki/Trill_(music) or https://en.wikipedia.org/wiki/Mordent for details.