What is KEI?
KEI - Keyword Efficiency Index is an estimate of the profitability of a given keyword. KEI is calculated with two parameters taken into account: # of searches and Competition.
A high KEI value (green indicator) reflects the keyword's high profitability (the term is searched for relatively frequently, while the competition is not too dense), and a low value (red indicator) suggests the keyword may not be worth the SEO hassle.
KEI factor is used in Rank Tracker to help you determine the quality of a keyword. The higher is the KEI value - the more visitors you may expect from that phrase should you occupy top positions on Google result page.
Low KEI would indicate that you may not expect a lot of visitors from organic search for that key-phrase, at the same time, he number of competing websites is relatively high and it may take a lot of errorts to outrank them.
You may also check this Glossary page to learn about other factors that are being used in SEO PowerSuite tools and their meaning.