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The eye socket is the bony compartment where the eye is kept.
Literal
Eye-socket [as-for-とは], eye [object-を] put-in bone [genitive-の] container [genitive-の] thing is.
An anatomical definition. 眼窩 (がんか) is the medical term for 'eye socket' or 'orbit,' borrowed from classical Chinese anatomical vocabulary. The ~とは...ことです frame is a slightly informal definitional style — ~のこと serves as 'the thing called X,' bundling a description into a nominal predicate. 目を入れている骨 is a relative clause: 'the bone that holds (puts in) the eye.' いれている is the ~ている form of 入れる ('put in, insert'), here meaning 'has [the eye] placed in it' as a resultative/stative. The whole sentence is a children's-encyclopedia style of anatomical definition.