As the name implies, during this time the experience gained from kills or spell/skill increases is doubled. This time period ranges from 10 minutes to 60 minutes and is commonly refered to as "double".
There are many ways for this event to occur. The most common being when someone uses the <superhero loud> command. (This is a ten minute double.)
Other instances of double are:
You can type 'double history' to see the last 50 double experience occurrences.