The expense ratio (ER) of a fund is the ratio of total annual costs to the value of assets.
The ETF VOO has an expense ratio of 0.03% at the time of this writing. That means you'll pay about 3 cents USD per year for every 100 USD of assets (S&P 500 stock) you own. Actively managed funds typically have much larger expense ratios, sometimes exceeding 2%, due to the large fees paid to the investment managers or advisors. Other expenses pay for recordkeeping, custodial services, taxes, legal expenses, and accounting. High expense ratios are typically bad – they are not correlated with higher returns, but in fact are correlated with lower returns after accounting for fees.
Investopedia's article on expense ratios
Wikipedia's article on expense ratios