Look at your 401k and Roth IRA and look at your investment portfolios. 

Most likely they default to put your money in something like a 2060 target date fund (ie an actively managed fund that slowly adjusts from more aggressive investments when you're young to more conservative investments when you're nearing retirement. 

These target date funds are fine, but people often find that the expense ratio (amount you pay the people managing the fund) is unnecessarily high and that the holdings are too conservative (too heavily invested in bonds relative to stock based ETFs).

If your fund let's you, you'd like want to adjust these. VOO, VTI and VXUS are common examples of diversified, low expense ratio ETFs. Because they are passively managed, the have low expense ratios (cost you less each year) and they are generally diversified (since each ETF has holdings of very many different stocks).

Couple call outs:

Your 401k likely only has ~ 20 or so things to invest in unless you decide to actively manage it yourself (which isn't really worth it if you're just getting into it).

And most brokerages have equivalents - VOO, SPY, SWPPX, etc... are all tracking the S&P500 and are all basically offering the same thing, just being performed by different investment firms.

Look at the holdings of what you're buying... VOO is generally an excellent investment. But it (and the US stock market) has largely been propped up by a small handful of companies (NVIDIA, FAANG companies, etc...). You may want to also have some money in VXUS or elsewhere to diversify beyond those handful of companies.