What it means
ETFs let you buy a whole basket in one trade. Some track the S&P 500, others gold, a single sector or even Bitcoin.
In practice
An ETF gives exposure to a whole market with one ticker. SPY and QQQ follow the S&P 500 and the Nasdaq-100, and others follow gold, bonds or a single sector. They trade during stock market hours like any share.
Traders use the big index ETFs to read overall market direction, and investors use them to hold a market for years at low cost. The most traded ones have very tight spreads.
What to watch for
Leveraged and inverse ETFs reset every day, so over weeks their return drifts away from the simple multiple of the index. They are built for short holding periods, not for buying and forgetting.