It means if a label is the first label of a day, it will include both the hour and minutes (hh:nn) and the day and month (mmm dd). Otherwise, it will just include the hour and minutes (hh:nn). By default, this API will "promote" the first label, that means the first label will always include the day and month. So even if your data has less than 24 hours, the first label will still have the date and month. The intention is to allow the user easily know the full time of the labels. The "promotion" feature can be disabled. (See the doc on the setMuitiFormat API.)
Another issue you may consider is how you can be sure date/month and hourly labels are the option possibility? If you let ChartDirector to automatically generate the labels, depending on your data, it is possible to generate per minute or per 30 minute labels if your chart contains 120 minutes of data. It is The Axis.setMinTickInc can be used to configure minimum label spacing for automatically generated labels.