When a pregnant woman's water breaks, what is actually happening is that?

Rupturing of amniotic sac is known as water breaking.

During pregnancy, the child is encompassed and padded by a liquid filled membranous sac called the amniotic sac. Commonly, toward the start of or during labor time, it will burst and this phenomenon happening in pregnant women is called water breaking.  

When the water breaks before initiation of labor pain it is called prelabor crack of films (PROM).  

At the point when the water breaks, the pregnant individual may encounter an impression of wetness in vagina or on perineum, a discontinuous or consistent spilling of modest quantities of watery liquid from vagina, or an increasingly evident spout of clear or light yellow liquid.