Find looks for NON-zero values. It returns the index of those elements, as they are stored in memory. Remember that elements are stored down columns, so all elements of the first column are first, then the second column.
A linear index is the index of what you would get if you unrolled the array into a long column vector.
If you give find a second return argument, then it returns row and column indices for the non-zeros.
By the way, this is stated in the help, so you could have learned this as easily and more rapidly, simply by reading the help for find. A good way to learn these things is by reading the help, then try some test cases of your own.
