As for your question then if you have an array named for example a then *a returns reference to its first element.
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In your example a is a three-dimensional array. Its element is a two dimensional array. That is a three-dimensional array is an array of two-dimensional arrays.
So *a is the first element of the three dimensional array.
{ {2,4}, {7,8}, {3,4} },
It is in turn an array the element of which is one dimensional array. So **a is the first alement of the two-dimensional array that is {2,4}.
***a will give the first element of this one-dimensional array (**a).
In your example it is 2.
***a + 1 is equivalent to 2 + 1 and will be equal to 3.