A simultaneous press involves pressing two or more buttons at the same time, or perhaps with a directional input, either required at that same moment (like with Smash Bros. “smash attacks”) or done before you press multiple buttons (like with KOF “emergency evasions”).
Some games allow you to stagger the input just a little bit; if you are off by a only couple of frames in one of KOF’s many simultaneous presses, for example, it will still register as simultaneous. This principle allows for Street Fighter III’s kara throws, for example.