This is a tuning fork much like the ones we used in Friday's lab.  It is impossible to see, but this tuning fork is resonating at 440 hertz.  This means that 440 cycles happen in one second.  If the frequency is 440, then the period is 1/440.  The note caused by the fork resonating is an A.  The reason I am pressing the tuning fork to the table is because sound travels through solid's better.  If I hit the tuning fork but did not put it against anything, I would only be able to hear it if I put it close to my ear.  Placing it against the table amplifies the sound because the table vibrates in tune with the fork.