What is this symbol called in music?

Mordent (a rapid-alternation of a note)

Explanation:

The lightning-bolt like symbol represents a rapid-variation of a note. There are two types of mordents. A regular mordent, and an inverted mordent.

How to play mordents/inverted mordents:

When playing a regular mordent, you rapidly play the written note, the note above it in that scale (which is determined by the key signature), and then the note written again. When playing an inverted mordent, you rapidly play the written note, the note BELOW it in the scale, and then the note written again.

Let's say an eight note with a regular mordent above it is D. Let's also say the key signature is C, just to keep it easy on us. In this case, you would play "D, E, D," except all those notes would equal an eight note beat.

With an inverted mordent, as stated already, it's the same thing, except you rapidly play the note below..

Example:

There's a quarter note A on a treble clef, with the key signature of F. We know that an inverted mordent plays the note below rapidly, meaning that you would play "A, G, A," with the notes all equalling a quarter note beat.

If this explanation isn't helpful enough, or you don't understand. I would highly recommend looking up videos on how to play mordents based on your instrument. But, I hope I did help those like you who were wondering.