Stars represent a players contribution to the game.
These are not to be used to compare teams but can be useful for comparing players who are playing in the exact same position. Also, it was the analysis of stars that helped experienced managers in determining the percentages of contributions for each position (e.g. 75% scoring and 25% passing for forwards).
Stars are evil
Stars are considered evil as they do not give a very accurate overview of a team actual strength although some people believe in them. If stars are used with care they are not "evil" as such they must simply be taken in context. Threads entitled Stars are evil often appear in conferences as people do not understand them.
In any case, the saying "stars are evil" is helpful as it helps noobs understand the fact that stars are next to irrelevant when it comes to deciding a match (so it reduces comments like "I should have won, I got 22 stars and he had only 20".