The Ethereal Plane is another dimension, described in more detail on Pages 48 and 49 of the DMG. It may help you to replace the term "plane" with "dimension." The ethereal plane borders several other planes, and is often described as a muted, foggy outline of whatever exists on the plane that it borders.
So, take for example that you are in a corridor inside of Castle Ravenloft. There is a spot in the ethereal plane which looks exactly the same as the castle corridor, except it's all fuzzy looking and grey and foggy. Any creature on the ethereal plane is invisible to the stuff in the castle corridor, but the creatures in the corridor are visible to the ethereal denizens. Creatures on the ethereal plane also feel no gravity, and can move through walls easily.
The ethereal plane is used very often then for travel, as well as espionage, since travelers can move through walls, as well as spy on people on the material plane without worry of being discovered. Ghosts and other such incorporial undead can move into this plane at will (which is what allows them to turn invisible).
If a character can use true sight, they can see into this plane. So if anything is spying on them using this plane, they will see it. They can also see ghosts and other such spirits that try to hide from them by moving into this plane. They will also see things that were hidden in the ethereal plane for safe keeping.
As for the effect of this on your campign, that really depends on how much you use alternate dimensions as a DM. If you don't use the ethereal plane for very much, the spell just means that the PC can follow ghosts and other such things, as you have noted. If you like to put spies in there, or other such hidden things "stashed away in the fourth dimension", then the PCs will notice those things. The effect is pretty minimal, since your players would acctually have to shift themselves into the ethereal plane in order to affect anything you put on that plane.
As noted by Andrewk, Ravenloft is filled to the brim with trapped ghosts. Even if they don't directly interact with the players, it can be a powerful narrative/aesthetic device for the PCs to see so many souls in the ethereal plane as they move through the castle.