Sorry if I sound uneducated - but what exactly is probiotics - I hear it used everywhere but totally clueless what it is and why we need it

Probiotics are the healthy bacteria that keeps our gut moving. They help in digestion and fight the bad bacteria. The food they feed on is prebiotics. I'm often confused by these 2 terms. http://www.parenting.com/article/ask-dr-sears-probiotics-during-pregnancy