Unity Script Reference:
Legacy Documentation: Version 4.3.0
Matrix4x4.this[int,int]
var this[int,int]: float;
float this[int,int];
this[int,int] as float
Description

Access element at [row, column].

Both row and column must be from 0 to 3 inclusive. A matrix is 4x4 array of numbers, and you can access the individual elements using this function.

Note the standard math notation - row is the first index.
var this[int]: float;
float this[int];
this[int] as float
Description

Access element at sequential index (0..15 inclusive).

A matrix is 4x4 array of numbers (16 numbers in total). You can access the individual elements using "flattened" index with this. The index is row+column*4.