This depends on what type of security you are trading. See the Mutual Funds section above for information about mutual fund pricing.
For equity and options trading, the execution price you’ll receive will be dependent on a number of factors. For many equities and options, the most recent price might be from seconds ago, though it could be minutes, hours or even days, for less liquid securities. Instead of relying on the most recent, last trading price, a better indication is the bid price and ask price. The ask price is often referred to as the offer price.
The NBBO (National Best Bid and Offer) is a consolidated quote that represents the highest bid and lowest offer for a security across all exchanges and/or market centers. It is updated continuously during market hours. Along with the bid price and ask price, there is also an indication of size, representing how many shares are willing to be bought (bid size) and sold (ask size) at those prices. For equities, the size indicated should be multiplied by 100. A bid size of 5 actually represents 500 shares willing to be purchased at the bid price.
If you are placing a market order (hoping to receive the next available price), the NBBO is an indication of the price you could receive. For buy orders, the best offer price is the best indication of the price at which an order is likely to be filled. The best bid price is the best indication of the price at which a sell order will be filled. However, if the size of your buy order is larger than the size available at the ask, you should expect that some of your order might execute at a price higher than the ask.
In addition, there are various market conditions that can cause orders to be executed at better or worse prices than the bid and ask. While the bid and ask price are displayed to investors and other market participants, there can also be non-displayed orders at, inside, or outside of the bid and ask prices. There is the potential that your order will execute against a non-displayed order that is resting between the bid and ask, which could improve your execution price. For orders placed prior to market open, Fidelity may wait for the primary exchange to open before commencing trading in a particular security. Also, in fast market conditions, there could be orders ahead of yours that deplete all available shares at the bid or ask, moving prices in or out of your favor by the time you place your trade. News events, market volatility, market outages, and other circumstances can all impact the execution price that you receive. You should always use caution with market orders as securities prices can change sharply.