Article: Just as their customers pay them for Internet access, ISPs themselves pay upstream ISPs for Internet access. An upstream ISP usually has a larger network than the contracting ISP or is able to provide the contracting ISP with access to parts of the Internet the contracting ISP by itself has no access to.

Now answer this question: What does an upstream ISP provide for an ISP?
access to parts of the Internet the contracting ISP by itself has no access to