Financial markets do not go up or down in a straight line. On the charts, you can see they move up and then pull back or come down slightly and then move up again in an uptrend, and the opposite when in a downtrend.
Pullbacks in an uptrend often come back down to areas of Support or Fibonacci levels before continuing to move in the direction of the trend, whereas pullbacks in a downtrend often pullback up to levels of Resistance before continuing to move in the direction of the trend.