The only difference between these two examples is that Plus[1, 2] evaluates to 3, while Plus[a, b] evaluates to itself.
Apply evaluates its arguments according to the standard evaluation procedure like most other functions in Mathematica.
In[1]:= Apply[Times, Plus[2, 3]]
Out[1]= 5
In[2]:= Apply[Times, Unevaluated[Plus[2, 3]]]
Out[2]= 6
See also Non-Standard Evaluation.
