Triangulation - you basicallly are using different forms of evidence/data (e.g. 1 article + 1 report/video) or data collected by different people to support your research. - This makes it reproducible/reliable or whatever....
The other thing is bassically reseaching somethng e.g. proving/disporving a statement (aka hypothesis) in a orderly/methodlogical way e.g. pilot study then a questionaire(primary method then secondary) etc... (I think your textbook explains all this)
SOURCE: I GOT A IN SOCIOLOGY