Why do these triangle symbols mean?
The Delta symbol can be read as "Difference". So for example in chart B, to move the motor Forward: IN1 > IN2 (And Difference > 15), the PWM number must be at least 15 higher for IN1.  

Also notice that in Chart A the motor runs from 15 to 255 (below 15 it does not run), so in chart B it shows that Halt is when IN1=IN2 or Difference &lt; 15.  

Backwards is similar to Forward except of course IN1 &lt; IN2, and the Difference > 15.