Hi
It just means that if you consider the identity function \(\displaystyle id_G,\) where \(\displaystyle (G,.)\) is your group, then:
\(\displaystyle id_G\ :\ G\rightarrow G\ :\ x\mapsto x\) is a morphism, that is \(\displaystyle id_G(a.b)=id_G(a).id_G(b)\) for all \(\displaystyle a,b\in G.\)
This assertion is quite trivial, since the equality above means \(\displaystyle a.b=a.b,\) which is obviously true.