Spirit Guardians are ethereal beings that guard a particular location or object. They may be summoned, bound to an object, or spontaneously manifest as the living try to remove them from their rightful place. In this way they confuse intruders and attackers by constantly changing positions and attacking where the enemy is not looking.
Spirit Guardians also seem to have protective qualities that deter intruders and trespassers because of their spectral appearance; which often manifests as the form of a large humanoid creature with glowing eyes , fearsome features, massive size, robes or armor made out of light (sometimes grey instead). They radiate so much energy within an area they are guarding that some people who enter will suddenly come down with nausea, cold sweats, chills or headaches. In other words, Spirit Guardians are a little more than just animating force fields.
Look at most of the spells that deal with ethereal beings, they tend to imply movement is not possible in most cases – or at least very difficult. Magic Missile is an exception but there are others like Detect Evil and Good , Zone of Truth , Arcane Eye , Arcane Hand et al. In other cases, there are ways to move in the spirit world that the caster is responsible for performing (for example Astral Projection ).
In D&D 5E, it’s important to note that ethereal movement is a magical effect and subject to dispel magic . It is not a spell in itself though some monsters can create a layer of etherealness on top of an existing solid structure (such as the Skum who make a lair out of Abyssal seaweed).