The five elements in Celtic tradition are similar to the five elements in other traditions: Earth, Air, Water, Fire, and Spirit. Of course, in the Celtic tradition, they are called by other names.
Earth, or Calas, is at the top of the equal-armed Celtic cross. This is the dirt, rocks, trees... all the substance we think of when we think of earth.
To the right, as you are looking at the cross, is Fluidity or Water. As the name implies, everything dealing with or living in water is represented here.
At the bottom of the cross is Air, or Breath. As the name implies, this element represents the wind, the breeze, even the very breath we breathe.
The final arm of the cross is Uvel, or Fire. In addition to actual fire, this element includes the sun, and heat, and even light.
The center of the cross represents Spirit, or Nwyvre. This is the element of the Divine that we, as humans, can not see or touch, but, if we are lucky, can sense.