Importance:
AWA
AWA explains that the "interest bar" represents the click-through rate (CTR) for that ad for that search query.
AdWordsAdvisor Says: [
Link to quote]
Vibgyor79 is correct. The interest bar is a graphic representation of the CTR for that ad, for the particular keyword searched.
If the bar is full, it essentially means the CTR for that ad/keyword is 1% or above.