Many people in the world have many dreams, but only a small subset of those are what I call "prophetic". The term "prophetic dream" can seem ambiguous and even nonsensical to a lot of people and can mean a lot of different things to different people.
I am using the term in a strictly Christian sense, deliberately ignoring what it might mean to people with different religious or philosophical backgrounds. I am using the term in the sense of the Christian "prophetic" of 1 Corinthians 14:3, which broadly speaking says that a prophetic word "encourages, comforts, and instructs".
This is contrary to the modern notion of a prophesy as a declaration of future events (most popularly, the end of the world). While that is a valid type of prophesy, it is only a subset of real prophetic utterances. Other people might question whether such prophetic words can be given through dreams, and if they are given, by whom.
I will answer that prophetic dreams are recorded in the bible and that all true prophetic words come from God, by the Holy Spirit. I will not justify this position for the sake of brevity.
So anyway, what I look for in dreams is encouragement [strengthening, direction, impetus towards some God-directed action(s)], comfort [solace], or instruction [teaching about doctrine, knowledge of future or present events/circumstances, knowledge of other people or myself].
Sometimes dreams can be what I would call a "prophetic experience" more than just a "prophetic word". This means that it isn't God filling me with knowledge about something, but I am actually experiencing events in the spiritual world, or experiencing a dream as a metaphor for some spiritual reality.
Since dreams are the gateway into the spirit realm, it makes you more open to receive prophetic words from the Holy Spirit. It also opens you up to travel into places in the spirit realm or to have encounters with other spirits (of people, angels, demons or God). There are some cases when I see angels in my dreams and they are figurative/symbolic images in the dream. There are other cases where I believe that I have real encounters with real spirits of angels. For instance, Joseph the father of Jesus encounters a real angel in at least two separate dreams.
One can very reasonably ask how to discern between the two cases, where a being is a real angel vs. a mental representation of an angel. I do not have a clear, hard rule to answer with. The best I can say is you need the gift of discernment (see 1 Corinthians 12:10, "distinguishing between spirits"). Discerning dreams is an area of a lot of confusion, understandably so. I do the best I can to understand what's real, what's symbolic, what's figurative, what's literal, what's my own emotions coloring things, what's a "pizza dream". I do this imperfectly, leaning upon the excellence of Christ to guide me.
Much of the time, I record a dream and consider it prophetic simply if it feels.... unusual to me. There is no hard rule to say what that is, because God does new things, not always predictable things.