In various types of video game genres (fighting, beat em up, action), there is a concept known as "combos". Combos are essentially a series of buttons that are hit in quick succession to deal as much damage to the opponent as possible. If you are executing the combo but not hitting the next button in a quick enough time, you have essentially "dropped" the combo and have to start the combo over again.
In this example, we have three combos you can potentially execute. If the key shows up on the screen on the press, then you are executing the combo. However while executing the combo if the input disappears, that means you either had the wrong input or didn't hit the button fast enough. You can change the "comboLinkBuffer" variable in the code to give you a longer delay.