I don't get the width and height part

What they're saying is that the first two bytes (first two eight bit segments) tell you the width and height of the pattern.

In the example given, you'll notice that the first 16 digits are 00000100 00000100.  If you convert those to decimal, you'll see that those are both equal to four.

If instead the second block of eight bits was 00000111, the image height would then be seven.