what dose the line "my array[0][2]= 42" mean?

Answer 1

it adds value of 42 to element in first row and 3. column of 2D Array

Answer 2

it means that we have changed the value of the element which is in [0][2]
e.g.
if there is an array [][] = {{5,7,4},{3,5,2}}
and then we use
array[1][2]=8 then the new array will be as below(changed)
array[][]= {{5,7,4},{3,5,8}}

hopecit helps

Answer 3

it means you are putting 42 at 1st row and 3rd column in 2D array.

Answer 4

It means assigning the value 42 to the element referenced by the indices in a two dimensional array of type int.