An aftermath is what happens after a big event or disaster.
When something big happens, like a storm or an accident, there's always some sort of aftermath. It's the mess that's left behind - broken things, emotional reactions from people, and maybe even economic problems because of lost work or damaged property. Think about it this way: when your favorite sports team loses a big game, the next day is full of disappointment and reaction to the loss. That's an aftermath. If there's been a natural disaster like a hurricane, the aftermath can be months or years of rebuilding efforts and people dealing with the trauma they experienced.
(obsolete, agriculture) A second mowing; the grass which grows after the first crop of hay in the same season; lattermath.
