Most day trading markets include three separate (but connected) market prices. These prices are the bid price (the current highest priced buy order), the ask price (the current lowest priced sell order), and the last price (the most recently traded price). The bid and ask prices are used in various ways to place and fill orders (market orders, limit orders, etc.), and the last price is used by charting software as the current price for the market in question.
Forex markets however, have a bid price and an ask price, but they do not have a last price. More accurately, forex markets have a last price (obviously there is a most recently traded price), but only the traders that traded it (and their brokerages) know what that price is.
Why is the Last Price Missing?
Day trading markets are usually provided by an exchange (such as the Chicago Mercantile Exchange (CME) in the US, the London Stock Exchange (LSE) in Europe, and the Hong Kong Futures Exchange (HKFE) in Asia). The exchange handles every order and every trade for their markets, and is aware of every trade that occurs, so the last price is well known. However, forex markets are not provided by an exchange. Forex trades happen directly between the two traders involved, and nobody else need even be aware that the trade has occured. This means that the last price is not generally known, and is therefore not available for use by charting software.
What Does this Mean for Trading?
The most visible result of the missing last price is that charting software must use a different price as the current market price. Some charting software uses either the bid price or the ask price, and some charting software uses the average of the current bid and ask prices. This means that different traders can be watching the same market (e.g. the EUR to USD), but can have very different charts (e.g. bars with different highs and lows, etc.).
If you trade the forex markets, make sure that you know which price your charting software uses for the current market price. By knowing which price is used, you can adjust your trading accordingly (e.g. using well placed limit orders instead of market orders), and receive better prices than traders who do not know which price their charting software uses.