Shadesmar is a plane of existence. It is the cognitive realm, and binds the other two realms of existence (spiritual and physical) together.
Shadesmar can be visited by those with an inherent Soulcasting ability, or those who have a Soulcasting fabrial. So far, the only known characters to travel there are Shallan Davar and Jasnah Kholin.
Most people only interact with Shadesmar subconsciously through intuition or artistic expression, but Shadesmar is all around everyone all the time. It is created by the deep, profound thoughts of people and then shaped by spren.
Lay of the LandEdit
Geographically, it appears to be a reflection of Roshar, with its inland seas corresponding exactly to the shape of the land of Roshar. Within it are:
Shallan describes Shadesmar as an endless sea of tiny black beads that appear to be made of glass. There, the sky is black and a small white sun above the horizon seems as though it is not far away. Above the sea of beads, flames hover "like the tips of candles floating in the air and moving in the wind".
Since Shadesmar is a representation of the cognitive realm, it exists in the same time and place as the physical realm. It's all around all the time, but people don't really see it because they are firmly rooted in the physical. It has been theorized that those people who do see parts, are typically only doing so subconsciously. Strong emotions and thoughts seem to make that easier. For example, when Shallan is drawing, she can see Cryptics because she is more connected to the cognitive realm while she is creating art. The same is true of Elhokar, who is so severely paranoid that he can see Cryptics over his shoulder when he looks into mirrors. They aren't actually 'in' the Cognitive Realm, they're just glimpsing it. Then, there's soulcasting. When Shallan soulcasts, she certainly 'seems' to be in Shadesmar; she's floating in the ocean of little beads, but she has very little control over what goes on, and is told by Jasnah that what she did is incredibly dangerous.
Suppose that most people who 'go' to Shadesmar don't really go there, per se. Even Jasnah herself says that Shadesmar is not really a place because it's the same place in which one already is, just a different representation of it. Most of the time, the difference is where one's consciousness is. Think about spren. Spren are from the cognitive realm, and that is where they really exist. Spren like Pattern look completely different in the physical realm than they do in the cognitive realm because it is unnatural for them to be in the physical. It is only their bond with humans that allows them the ability to think outside of the cognitive realm, just like the bond with the spren allows humans to have consciousness in the cognitive. Still, in both cases, neither is actually fully 'there'. That's why spren can't really move things in the physical realm, and it's why Shadesmar is so dangerous for people. While Shallan 'sees' and 'feels' Shadesmar, her body remains in the physical realm. When soulcasting, people only travel to the cognitive realm mentally, not physically. This is known because neither Shallan nor Jasnah disappear when they soulcast; only each their consciousness shifts completely into the cognitive realm, not their bodies.
However, Jasnah is an Elsecaller. While using one of her two abilities (i.e.: Transformation), she is just glimpsing the cognitive realm the same way that Shallan does, in order to soulcast. Her other ability is Transportation, though, and that allows her to actually, physically go to Shadesmar.
The difference is made visible when Hoid is waiting for Jasnah and she suddenly appears. She was fully in the cognitive realm and then returned from it to be fully in the physical realm. This is something that Hoid, as a Worldhopper, can also do (and without a bond with a spren).
So, Jasnah may have faked her own death in two parts: first, she soulcast her blood to save her life after being stabbed. Then, in the confusion, she transported herself to the relative safety of the cognitive realm and stayed there until everyone was sure she was good and dead.