Let $\displaystyle f,g:R \rightarrow R$ be Lebesgue integrable functions. Let $\displaystyle \phi(x,y)=f(x-y)g(y)$. Show that $\displaystyle \phi$ is well defined.

i am not sure what it means by a function is well defined. would someone tell me what i have to show to say that it is well defined? any help would be appreciated.