Latency is the delay before the data transfer begins once the instruction for the transfer arrives. When the data travels over the network, getting the request by the Subscriber and then sending the expected data back takes time, as the server needs to first find the data, then build the data, and later send it. This process is obviously going to take more time, and the time taken will be increased if the requested data is more. Thus, the application will take more time to respond.
Let's take a very simple example from our day-to-day life to understand latency easily. We all browse the internet. A few pages which we visit get loaded very quickly and a few take more time. Have you observed the general pattern where the time taken by the browser is more? These are the pages with lots of images and graphics.
Now, the question is, how will latency affect reactive applications...