Yes WTF is triangulation. Sounds mathematical and more suited to an airline pilot than an occupational therapist.
In one sense it’s using points in a triangle shape to pinpoint a location more precisely, but the sense we’re concerned about here is the social science sense. Really simply it’s using two or more sources of data to make the results more valid.
The sample is the people you will experiment on in your research to obtain results. In an ideal world the study should examine an unbiased representative mini-chunk of the actual population, so that the results can easily be extrapolated and applied to the real world. To get your sample as close to the real world as possible, you must consider:
who should be in your sample?
the ethics of all this experimenting on the poor sample people