In a physical sense, the word "fallout" means the radioactive particles that are in the atmosphere after a nuclear explosion, and then "fall out" of the sky, back to the ground, usually in the form of dust.

However, in your sentence, the word "fallout" has a figurative meaning:  the bad consequences that are created as the result of a situation.  Example:  "If you criticize your friends behind their backs, you will have to accept the fallout (the bad consequences) that will occur when they find out what you are doing."   [In this example, the fallout might consist of the loss of friendships, and your acquiring a bad reputation as a backstabber.  A "backstabber" is a person who says or does harmful things about people behind their back.]  So in your sentence, something is "going on" which has caused unpleasant consequences (fallout).