ETF stands for an Exchange Traded Fund. That means its value can fluctuate whenever trading can occur. Many of them are tied to various indices, but because they are ETFs, the value of that specific fund could be greater or lesser than the index they are tied to. In fact, an ETF does not even have to remain in sync with the net values of the assets the ETF itself holds. E.g. an ETF can trade at $50/share even if each ETF share is equal to holding 3 shares of companies each trading at $10.
I suspect that you are thinking of a mutual fund which is required to be traded at its component's net asset value (NAV). In the example above, a mutual fund has to be traded at $30/share because its 3 constituent holdings sum up to $30. This is also why mutual funds are only exchanged once per day at the end of the trading day, so that its most accurate NAV calculation can be completed. Because ETFs don't have to be traded at NAV, their price can go up and down as the market dictates.