Generally speaking, dreams are not particularly significant and should never be used as a guide for spiritual direction, certainly not in a way that would contradict the plain teaching of Scripture. In the Bible and in the progress of revelation, God revealed Himself to the prophets through dreams and visions to communicate His truth. Speaking of this and contrasting it with the final revelation through His Son, the author of Hebrews wrote:
God, after He spoke long ago to the fathers in the prophets in many portions and in many ways, in these last days has spoken to us in His Son, whom He appointed heir of all things, through whom also He made the world (Heb. 1:1-2).
The “many portions and ways” refers to the various means God used to communicate His message to His people, but with the coming of the Son we have God’s final revelation that has been passed on to us through the Lord Jesus and His disciples as He promised in John 16:12-15. See also Hebrews 2:1-4. This completed revelation is found in the Scripture, the faith once and for all revealed (Jude 3). I do not believe God speaks to us today through dreams, certainly not as He did in the Old Testament so that what is dreamed becomes on a par with the Bible.
Having said this, I do think that God uses other avenues (people, trial or difficulties, circumstances, etc.) to teach us and call things to our attention that we need to deal with or learn from. If after climbing a set of stairs I am huffing and puffing, I can know from this that I am out of shape and need to make some changes in my lifestyle. In this way, the Lord uses circumstances to teach us. So likewise, dreams may be significant if one is consistently having certain dreams. Emotionally and spiritually this could suggest a spiritual problem that needs to be resolved such as in a family relationship. Perhaps the subconscious is expressing itself in one’s dreams. One would then need to examine his affairs or relationships and attitudes to determine if there was something that could be causing his or her dreams. The solution would be to deal with the possible causes through the application of biblical principles and promises. But I would not be concerned about an occasional nightmare. Some people, if they watch a horror movie, will invariably have bad dreams sometimes for several nights after the movie. This very fact does show us that what’s going on in our subconscious (fears, anger, frustrations) can promote certain kinds of dreams.
In general, our focus should simply be on knowing and relating to the Lord through His Word. “Draw near to Him and He will draw near to you” (Jam. 4:8a). Resting our hearts on the Lord will deal with our subconscious, give us peace, and in general give us good dreams. I have heard of people who have had dreams of something happening to someone they knew and it actually happened. This is rare, but certainly God is able to do this. More often than not such dreams do not come true and are usually the product of inner fears or worries.
There is also the possibility of a demonic cause of bad dreams, but generally such only occurs if one has been dabbling in the occult. I have heard of people experiencing horrible dreams when they have gotten involved with drugs, the occult, Ouija boards, and seances, etc. Terrible things can begin to be experienced as a part of demonic influence and harassment. In such cases the only way to stop this is to turn to Christ, confess this as sinful, reject it, and turn from it by a positive pursuit of knowing and walking with the Savior.