This is an eighth rest.
It indicates half a beat of silence.
Using eighth rests on the beat is a common way of creating syncopated rhythms.
Here is a song using eighth rests. Remember to count in your mind to keep time while you are playing.
This is a natural sign. A natural sign cancels the effect of a sharp or flat for the rest of that bar or until another sharp or flat sign occurs within that bar.
This song contains sharp, flat and natural signs.
Here is a 12 Bar Blues containing sharps, flats and naturals.