Away & Frost only last to the end of the day. It shouldn't happen in Holiday though. We suggest you putting a low temperature schedule into the app so it never goes on.
Standby, will put the heating back to its "setback" temperature until the next change point in your schedule. This would be used if you want to bring your schedule forward (ie it's 2PM, and you have a schedule for 5PM to heat at 22C). Or to turn off your heating for the 'on' period you are in.
For the Away mode, this turns your heating to the setback temperature for the entire day rather than until the next change point. When the next day arrives at 6AM, then the schedule will resume. This is to be used if you know you'll be away out to a restaurant, pub, shopping etc for some time - standby would have the system revert to schedule at the next change, whereas Away will make sure it stays cool until you cancel it or a day change occurs.
Holiday mode, this is where you specify the number of days you would like to be away for, then the system will remain at the temperature you specified for the days you set.
The standby option in the app will toggle between states; so it will switch the Thermostat to the standby or scheduled state. You should find that if you were to press the button again it could change to the standby state.