Simply put, it means to stop something, typically a recurring action, from happening. It's a metaphor where tide refers to ocean waves, which are persistent. As the tide comes in, the water level rises. Stem typically refers to plants, but in this context it's used idiomatically to mean "stop".Thus, literally speaking, if the water level is rising and you stack up sandbags to prevent a flood you are stemming the tide. Used metaphorically, "to stem the tide" can refer to any kind of rising trend. For example, "we need to stem the tide of economic inflation" or "stem the tide of malaria cases". The phrase is usually used to refer to something perceived as negative. You wouldn't say "to stem the tide of college graduates" or "stem the tide of happy marriages" unless you're perhaps a cartoon villain. I hope this helps. --Q

stem the tide( of something ) = to stop something from increasing > This law may stem the tide of pollution of our beautiful river from the factories built along its banks. From Online Dictionary