"On Hold" is a gesture, and it's a triggered event. It means that it happens only once, and after you've kept your finger on the screen (in the same spot) for about 1 second.
"Is touching" is a condition, this event will be executed on every tick from the moment your finger touched the screen and as long as it continues touching, even if you are moving it.
So "On hold" is good for controls like menu buttons, when you only need to execute the action once. "Is touching" is suitable for things like virtual joystick etc.
It's important to remember that all "On ..." events in Construct are triggered, and all "Is ..." events are continuous, and to understand the difference between them.