Tape diagrams are a way to represent ratios. If you are given the ratio of 2 apples:3 oranges you would draw two boxes and below that 3 boxes (each box should be the same size).
If you were then told that you had a total of 100 apples, you would have to split the 100 apples into each of the two apple boxes evenly. You would then know that 50 peices of fruit go in each box (there would be 150 oranges total.)
Hope this helps!