What exactly do the variables named "Unemployment rate, via Census data" and "Poverty rate, via Census data," found in the College Scorecard data dictionary, represent?
Do these rates represent the unemployment rate/poverty rate among the student body, among the graduates of the institution, or in the region in which the institution is located?
These variables are calculated averages of poverty and unemployment rates. The calculation is based on census rates for each individual student based on the ZIP Code provided in his/her FAFSA application. The applicable rates for each student are then averaged to get the institution-level rates. These serve as a proxy of percentage of students comimg from low-income areas.
Where did you get this data and/or data dictionary?
Probably the variables represent, respectively, the unemployment rate and poverty rate, and the source of data is the US Census (as opposed to other methods of gathering information about unemployment and/or poverty rates).
https://collegescorecard.ed.gov/assets/CollegeScorecardDataDictionary-09-08-2015.csv. See also: collegescorecard.ed.gov/data/documentation (There's an explicit "Join the Conversation: StackExchange" link on that page)