Can not understand idea of the KMP algorithm
Sedgewick continues: "These are 
pattern characters that we know, so we can ﬁgure out ahead of 
time, for each possible mismatch position, the state where we 
need to restart the DFA."

What I understand is that this quote tries to explain that if you encounter a mismatch at letter at position $j$ of the pattern you have to shift the patterns by one position against the text and see where that leads us. The point there is that we do not need to know the actual text at that moment, because we know that so far the patterns has matched the text and we can match the pattern to itself (shifted).