Whose code is it, anyway?
Every package you use comes with a licence, and some come with obligations. This is general guidance, not legal advice: ask your legal team when unsure.
Permissive
MIT · Apache · BSD
Use it freely. Keep the licence notice.
Copyleft
GPL
Distribute your software, and you may have to share its source.
Network copyleft
AGPL
Obligations can apply even if users only access it online.
No licence
All Rights Reserved
Public on GitHub isn't permission. Don't use it.
Sources: GNU ↗ · choosealicense.com ↗ · US Copyright Office ↗
Check the licence. Ask legal when unsure.