Literally: heaven – change – earth – different
Alternately: Natural disasters or bizarre natural phenomena. Bad or weird stuff that’s not man-made. You know, like eclipses, record-breaking storms, and floods.
Notes: 天変 refers to unusual celestial events like meteors, comets, and eclipses, along with bizarre or drastic weather such as tornadoes, torrential rain, and powerful storms. 地異 refers to terrestrial events such as earthquakes and tsunami, floods, and volcanic eruptions.
Apparently some people replace 天 with homophone 転, but this is considered an error. On the other hand, doubling the 変 to get 天変地変 (~chi.hen) is a valid alternate version.