Before computers and the internet made everything awesome, financial trades would have to be conducted over the phone. This wasn’t a huge deal most of the time, but it did result in there being a lag between when a stock price was announced and when folks could actually trade based on that price.
In other words, trading wasn’t really done in real time the way it is now, and has been since pretty much the late-1990s.
These nearly-instantaneous price updates are known as flash prices, since they are updated at the speed of technology and can be traded on just as quickly.
Is there still a tiny bit of lag? Yes, there is, but it’s not nearly as long as it was before the advent of computerized trading platforms. Thanks, internet.