In Steam the play time of a person is displayed in a decimal formatted number such as 0.5 hours or 21.9 hours. what is the significance of the number after the decimal point? And how should these times be read in the usual hours and minutes format?
You just need to calculate the minutes from the decimal part via this calculation:
60 * [Decimal part]
So for your examples:
0.5 hours: 60 * 0.5 = 30 minutes 21.9 hours: 60 * 0.9 = 54 minutes (21 hours, 54 minutes in total)
Why they don't just put it in hours and minutes, I have no idea! Seems a bit strange really, especially then you have under 2 hours playtime on a game and it reports that in minutes only.