Solstice means "sun stop" ("sol"= "sun" and "stice"="stop".  It comes from the fact that the sun sets in a different place each night (and rises in a different spot each morning) during the year.  As we head toward December 21, the sun will set farther and farther to the North as I look out West from my house.  After December 21, the point at which it sets will move back to the South until June 21.  The points at which it stops changing one direction and begins heading back the other direction are called the "Solstices."  Look at the explanation of the seasons for more information on the way the relationship between the earth and sun causes this effect.