It means that everything that exists has its reason, and everything that exists can find its reason.

Hegel's famous saying comes from his Principles of Philosophy of Right (Grundlinien der Philosophie des rechts 182).

the original text is: wasvernü nftigist, dasist wirklich; Undwas wirklich, dasist vernü nftig.

Extended information:

Hegel believes that the origin of the universe is absolute spirit (. It has everything freely, and then externalizes into nature, human society and spiritual science, and finally returns to itself at a higher level.

Therefore, everything on this development track is rational, that is, it is inevitable and realistic.

the reverse is also true. This is the original meaning of "being is reasonable". Numerous phenomena conform to a certain "reason" and have their own reasons, but they are not necessarily reasonable in the usual context, that is, "correct", such as crime.

Reference: Being is reasonable-Baidu Encyclopedia