Maybe some examples will help. In the picture below, I've drawn some resonance structures for butadiene and acrolein. What do they tell us? They suggest that the central "single" bond should have some double bond character, it's more than a single bond, but less than a double bond. By the same token, the "double"
bonds at the ends of the molecules should both have some single bond character. These are examples of fractional bond order. The real world consequences of fractional bond order is a higher barrier to rotation about the central "single" bond in these two molecules than we might have expected, and a lower barrier to rotation in the pi bonds than we might have expected.