IPA: /ˌsʌb.kəˈlɛk.ʃən/
KK: /sʌb.kəˈlɛk.ʃən/
A smaller group that is part of a larger collection.
The library has a subcollection of rare books that are not available to the public.
Subcollection → It is formed from "sub-" (meaning under or below) and "collection" (from Latin "collectio", meaning a gathering or assembling). The word "subcollection" refers to a collection that is part of a larger collection, thus being under or below it in hierarchy.
Think of something that is a smaller 'gathering' ('collection') that is 'under' ('sub-') a larger one — that's what a subcollection means.