What does Ezekiel 1:5-14 mean?

And from the midst of it came the likeness of four living creatures. And this was their appearance: they had a human likeness, but each had four faces, and each of them had four wings. Their legs were straight, and the soles of their feet were like the sole of a calf’s foot. And they sparkled like burnished bronze. Under their wings on their four sides they had human hands. And the four had their faces and their wings thus: their wings touched one another. Each one of them went straight forward, without turning as they went. As for the likeness of their faces, each had a human face. The four had the face of a lion on the right side, the four had the face of an ox on the left side, and the four had the face of an eagle. Such were their faces. And their wings were spread out above. Each creature had two wings, each of which touched the wing of another, while two covered their bodies. And each went straight forward. Wherever the spirit would go, they went, without turning as they went. As for the likeness of the living creatures, their appearance was like burning coals of fire, like the appearance of torches moving to and fro among the living creatures. And the fire was bright, and out of the fire went forth lightning. And the living creatures darted to and fro, like the appearance of a flash of lightning.

English Standard Version (ESV)