dream, is it illusion or real?

During pharmacology class, we learn about Sedatives and Hypnotics drugs, which leads me to this term REM ( Rapid Eye Movement), NREM (Non Rapid Eye Movement) and RAS (Reticular Activating System). It catches my attention and a lot of hands were raised, throwing questions and there was this person asking about dreaming phase.

Pn Nirmala explained that dreaming occurs mostly during REM phase, when we are not fully enter "deep sleep" and we still can remember most of the dream vividly. But there is also dream that occurs during NREM, the phase that we are fully into "deep sleep" and we can't really remember our dream.
I'm not someone who really think that what I've dreamed is actually happening sooner or later. Dreaming is an involuntary action, and none of the things happened in my dream are true.

Few days ago, I dreamt about my late father. My sister and I went grocery shopping and suddenly I noticed that Dad is with us too. I'm utterly in  shock, as far as I can remember, he's not here with us anymore.I remember he was gone. I asked him something about his existence, maybe he knew that I'm puzzled about his sudden appearance, and he said that I don't have much time with you today.

Then, we went somewhere.Dad leading us to his car, and it was a new car, not his maroon Avanza that he used to drive at home. I'm amazed, I think he must have a good life in heaven since he bought a new car.I don't quite remember what type of car it was.I tried so hard to re-collecting every piece of memory but I failed.  I felt very real. It's like he is alive and for that moment, I can convince myself that he's alive. Then he said he don't have much time, I realised that it was just temporary.

I woke up, feeling a little bit lost after what I've been dreaming. I called my mum later in the following day, re-narrate my dream to her.She told me that from the day he was gone, my mum never dream about my late father. We don't know why but mum said, he don't want to keep ourselves dwelling about our lost, she said also that it is a sign that he's happy with God now.
I told her about late dad saying he don't have much time.Mum explained to me maybe he just paying a visit whether you are doing good in your life.  I feel like crying because I know deep inside my heart I miss him so much.