Children learn to match the sounds they have been hearing
       (phonemic awareness) to the letters we teach them in school (abc's).

This is called learning the alphabetic principle. 
      They are, in a sense, learning their letter/sounds. 
           Some call it phonics. 
                We learn to use certain letters in order to spell.  

A grapheme is a SYMBOL we use to represent a certain sound. 

For instance:

                Example:  knife
                kn = grameme for the sound /n/  using 2 letters (k and n) to make it (the grapheme / symbol).
                  n = letter representing the sound /n/

                Example:  track
                 ck = grapheme for the sound /k/ using 2 letters (c and k) to make it (symbol).
                    k = letter for the sound /k/
                    c = letter for the sound /k/
                    c = letter (also) for the sound /s/
                    s = grapheme for sound /s/
 
                Example:   box
                      x = sounds /k/ plus /s/
                      ks = grapheme for the letter x (which needs 2 letters to make the symbol for the x sound)