Could someone give a precise explanation?
(Please answer with fluent English. Chinglish is rejected!)
What's the meaning of 'adjacent numbers on the row below'?

it is because the triangle is assumed as
1
2 3
4 5 6
7 8 9 10
so 5 is assumed to be connected only to 8 and 9
similarly 6 is only connected to 9 and 10 i.e. the "adjacent"
but
indexing is done as :
1
2 3
4 5 6
so adjacent to 2 are 4 and 5 and their indices are
0,0
1,0 1, 1
2,0 2,1 2,2
hope you guys get it now.