Two events are dependent if the outcome of the first event affects the outcome of the second event, so that the probability is changed.
Suppose we have blue marbles and red marbles in a bag. We pull out one marble, which may be blue or red. Now there are marbles left in the bag. What is the probability that the second marble will be red?
It depends. If the first marble was red, then the bag is left with red marbles out of so the probability of drawing a red marble on the second draw is . But if the first marble we pull out of the draw is blue, then there are still red marbles in the bag and the probability of pulling a red marble out of the bag is .The second draw is a dependent event. It depends upon what happened in the first draw.