The KEI (Keyword Efficiency Index) is a score between 1 and 100 that indicates the efficiency of a particular keyword for a particular app. The KEI takes into account both the Volume (Search Popularity) and Competition indicators.
Higher the number means better the balance between the Volume and the Competition. Keywords with higher Volume and low Competition will be more efficient and therefore get a higher score (= higher KEI).
The KEI should be interpreted carefully though. Although it is important to try finding keywords with the highest KEI possible, it should not be the ultimate goal. Relevance and Rank are both essential factors that need to be taken into account first.
Tip: You can effectively use the KEI in order to compare two keywords you're hesitating with (two synonyms for instance). Pick the one with the highest KEI possible.
The app's popularity/size also determines the importance of the KEI in comparison to other factors. A smaller app needs to pay a higher attention to the KEI and the Competition indicators while big brands may aim at ranking for high Volume keywords in priority.