I think the answer would be Idiom.
Answer:
C=Y, S=O, Y=U
you
H=D, I=E, G=C, S=O, H=D, I=E, H=D
decoded
Q=M,I=E,1=!
me!
Explanation:
a noun
jack and milly came to tea
jack is he,milly she and little susie? well that's me
To indicate that they are nouns