Full Idea
The basis of Hegel's system is that what is incomplete must not be self-subsistent, and needs the support of other things; whatever has relations to things outside itself must contain some reference to those outside things in its own nature.
Gist of Idea
For Hegel, things are incomplete, and contain external references in their own nature
Source
report of Georg W.F.Hegel (works [1812]) by Bertrand Russell - Problems of Philosophy Ch.14
Book Reference
Russell,Bertrand: 'The Problems of Philosophy' [OUP 1995], p.83
A Reaction
This leads to the idealist doctrine of 'internal relations'. It has some plausibility if you think about the physicist's definition of mass, which has to refer to forces etc. Presumably there is one essence for all of reality, instead of separate ones.