The prices that most people see - on a stock market ticker and on most general websites - are in fact what are called delayed pricing.
You might see a price for Telstra stock and assume that is the price that's being paid right now on the stock exchange. In fact, this is delayed pricing, where the trades that went through at that price may have in fact occurred 15 or 20 minutes ago. Delayed pricing is in fact the norm for most reports about the stock exchange, and for many investors, and for many stocks; a short delay in the price is not a huge obstacle to effective trading.
Unless major news intervenes, many larger companies trade within a fairly narrow band of 1 or 2 percent movement across the course of a day, and so delayed pricing by 15 minutes is unlikely to make a major difference most of the time. However, there are circumstances where delayed pricing might have a big impact. One example is when major news, for example, an interest rate rise that hasn't been predicted by the market, is announced, and prices move very suddenly. You might be relying on delayed pricing from 15 minutes ago where the market is now paying quite a lot more or less for your chosen stock because of the news. A more common example of challenges with delayed pricing is the example of smaller companies where because there's less trading and volume, prices can move very sharply with a small number of trades. In this case, delayed pricing might again show you a price that was in play before the most recent trades, and so you might be making trading decisions on out-of-date information.