The Doomsday Clock is a symbol which represents the likelihood of a anthropogenic global catastrophe. The clock represents catastrophe as "midnight" and the Bulletin's opinion on how close the world is to such as a number of "minutes" (and recently "seconds") to midnight.
Its original setting in 1947 was seven minutes to midnight. It has since been updated up and down according to the world events of the time. In recent years, the increments have been shrinking as the clock gets closer to midnight. The last update as of writing (in January 2020) shaved only 20 seconds from the countdown, setting the clock at 100 seconds to midnight.