This should be confirmed by your offer letter. Indicators that summer participation is required include 1) if your program start date is during the summer (i.e. before August), or 2) if you are receiving a stipend during the summer months (i.e. more than 9 months out of the year).
A dependent is either a legally married spouse or a child under the age of 21, who requires an F-2/J-2 dependent visa.
Please note that you will be asked to provide more information about each dependent, and to upload copies of the passport ID pages and marriage certificate for spouse and birth certificate(s) for child(ren), in the OISS Portal (http://oiss.rice.edu/oissportal).