Whether it be for the purchase of an ETF or a share, you as the investor have the option of placing your trade as a market order or a limit order. A market order is a trade in which you are agreeing to bid on whichever price the market is willing to pay. A limit order is a trade in which you determine the price you are willing to bid.
Market orders can pose a slight risk during purchasing if there are many of the same orders being placed at the same time, relative to the demand on offer. Due to the large influx of orders, your trade runs the risk of being filled with the next best market offer, which can potentially be a higher price than what you first thought.
Limit orders are often classed as the safer alternative, as you decide on the exact price that you are willing to pay. However, the downside of this is that you may run the risk of your order not being filled immediately.