The core idea
An index tracks a defined market segment. A fund may try to follow it, but you cannot buy the index itself directly.
A little more context
Fees, trading, taxes, and replication choices can make fund returns differ from its benchmark over the same period.
What to remember
A clear objective and an understanding of holdings and risks matter more than chasing a recent return.
Pause and reflect: how would you explain this idea to someone who has never invested?