What many call "dream walking" or "astral projection" is an advanced ability to disassociate from your surroundings either while sleeping or meditating. It starts with what is known as "lucid dreaming" - the ability to know you're in a dream and control what occurs - and advances to purposefully entering spaces, leaving the body behind at rest, to discover themes and messages that otherwise would have been missed to someone not trained on noticing even the most minute of details. My family has always referred to them as "striders."
Striders often gain this ability at the cost of traumatic experiences, particularly cases of child abuse and domestic violence. In these instances, the victim becomes hyper aware of their surroundings, people's moods, etc. in the hopes of changing their own actions to avoid receiving abuse. These people are often referred to "Empaths" when they are not able to stride. While this is a futile effort, the learned skill is key in striding successfully - a strider with no care for detail can give inaccurate interpretations to their own dreams or those of others.
A practicing strider can inhabit the dreams of others, either on their own accord or by request. The latter is preferred, as it is much easier to enter someone's dreams and not cause adverse effects. Also the former is just rude. /shrug
We do so in the hopes of answering pressing questions, determining someone's state of mind, or calculating the likelihood of future events. In a way, it's similar to the more common practice of "tarot reading" and Striders often use cartomancy as a means of starting the conversation prior to a stride. Striders may also revisit dreams when confused about a message to find missing details.
Striding is both physically and mentally tolling, and as such should only be completed in short bursts. Striders often unintentionally roam their own dreams or those of others, but will choose to remain passive in lucid dreams. On particularly sleep-heavy nights, we get migraines, bed sores, and wake up disoriented and lost. In some cases, it's difficult to walk in the "real world" without dragging in elements from the dream world.
It is also dangerous, in a sense, for our own mental health to stride into another's dreams. As humans we introduce parts of ourselves that we think others will approve of, and to stride is to learn the truth. We never know in advance the state of one's mind, and striding into the dreams of a disturbed or mentally ill individual can have adverse effects on our own mental health.
You've probably been told at some point not to wake someone who is sleep walking - not only would it put them in more danger if they were in the middle of a dangerous act, but it would put them into an intense state of shock. The same is true for striders - waking us can put is into shock but it can also erase certain elements needed to interpret the dream itself.
While I can't speak for others, my sleep state while striding has gotten so deep that I cannot be woken by outside forces (alarms, people, etc.) until I choose to wake up. This may in part be due to increased amounts of trauma in my lifetime, or an advancement in the skill. It is dangerous to stride without some tie to the world your body is laying in. If possible, stride in very controlled conditions where an emergency can be handled even if you are not awake to be aware of it.
Fortunately, we all dream in quick motion - a dream that may last hours or days in theory of the events will only last anywhere from 30 seconds to 10 minutes in real life. Don't believe me? When you have a vivid dream, immediately write down the dream or describe it in detail to another person. It'll be a super quick conversation.
There are many blogs out there that will give you "instructions" on how to stride / dream walk or learn to do it for the first time. These instructions tend to end at "be in a calming environment, state the intention aloud, and focus on the topic." I'm not sure how effective they are in making new Striders, but we all have certain environments we like to fall asleep in or imagery to think of to start a stride.
That's my spiel, if any other Striders want to chime in feel free!