The price of a traditional mutual fund is determined by something called the Net Asset Value (NAV). Each day after the market closes, this value is calculated by adding up everything that the fund owns and dividing by the number of shares. This NAV is then the price that you can buy into the fund for.
ETFs are mutual funds that trade like a stock on the stock market. ETFs also have a Net Asset Value, but the purchase price of the ETF fluctuates throughout the day as shares of the ETF are traded, subject to supply and demand of ETF shares. However, there are firms called Authorized Participants that are allowed to create or redeem ETF shares for shares in the underlying stocks. When the price of the ETF falls below the NAV, Authorized Participants can buy ETF shares and convert them to shares of the individual constituent stocks, propping the ETF share price up. Conversely, if the price of the ETF goes above the NAV, the Authorized Participants can convert shares of the constituent stocks into shares of ETF and sell them, bringing the ETF price down. In this way, the ETF price never gets too far from the NAV.