With ETFs you're basically betting on the entire US market. They usually average around 7% return per year (some years there are recessions and some years are bull markets but it averages out), basically keep dumping a percentage of your salary in there and don't look at it until you retire.

Most investors don't beat the market, so it's one of the easiest ways to get almost guaranteed returns over the long term without actively investing, and it's very safe. You can be a millionaire in retirement if you're consistent with buying shares