It means that theres always good that comes out of a bad situation. That everything is going to be okay in the end.
After a long period of darkness aka bad situation something great will come ( the light) a good thing will happen or a good idea will come
'Light at the end of the tunnel' is an idiom representing hope after difficulties. It conveys that despite challenges, a positive resolution is on the horizon. The phrase is commonly used in personal and broad contexts to signify impending relief or success.
;