These components are called subgraphs. Lets examine an example. Notice that the graph is composed of two smaller graphs which are not connected to each other. If you draw a vertical line between the 3rd and 4th columns, and a horizontal line between the 3rd and 4th rows, it splits the matrix up into 4 smaller 3 by 3 matrices. Here is another disconnected graph.
Lets see if the same thing happens again.
However, we can interchange vertices to transform it into a block diagonal matrix. First we interchange E and F, the 5th and 6th vertices, then C and F, the 3rd and 6th vertices. The result is a block diagonal matrix where each block is 3 by 3.