It's difficult to really understand what the person is saying
It's difficult to truly understand what the person is saying
人の話をちゃんと理解しながら聞くのは難しい
It's difficult to really understand what the person is saying
ちゃんと理解する=truly understand/ really understand
適当に相槌うつのではなく- instead of just nodding along.
上記を組み合わせると、
適当に相槌うつのではなく人の話をちゃんと理解して聞くのは難しい
It's difficult to really understand what the person is saying instead of just nodding along.
となります。