Triangulation means formation of triangles. In DNA genealogy we would analyze if A (you) would match both B and C (your matches) at the same location & chromosome AND that B would also match C at the same location. Example:
A matches B at chr 1 from 83,561,856 - 154,688,278
A matches C at chr 1 from 83,752,670 - 154,691,672
B matches C at chr 1 from 88,470,251 - 116,916,652
All 3 of them form a triangulated group at chr 1 from 88,470,251 - 116,916,652 as this is the area where they all overlap.