From Merriam Webster:

: a god introduced by means of a crane (see crane entry 1 sense 3a) in ancient Greek and Roman drama to decide the final outcome

: a person or thing (as in fiction or drama) that appears or is introduced suddenly and unexpectedly and provides a contrived solution to an apparently insoluble difficulty

Basically, it was used by the Ancient Greek playwrights as a plot device. The heroes would get themselves in a terrible mess with no way out then the gods would be dropped into the scene by cranes and solve everything.

It is what we need right now!

See also Deus Ex, the video games: https://en.m.wikipedia.org/wiki/Deus_Ex

In medicine

In medicine, the phrase is often used for supposedly "magical remedies" which are not likely to work in practice. For instance, in the 2020 COVID-19 outbreak, when double lung transplantation for terminal COVID-19 patients was suggested, it was immediately denounced as a deus ex machina.[22] In 2006, when electronic fetal heart monitoring was being touted as a preventive measure for cerebral palsy, The New England Journal of Medicine, denounced it as deus ex machina.[23]

an improbable or unexpected device or character that suddenly appears to resolve a situation

Expect a surprise plot change.