Reincarnation

One of my favorite verses from the Bhagavad Gita (and also one of the most popular ones) goes-

यदा यदा हि धर्मस्य ग्लानिर्भवति भारत ।

अभ्युत्थानमधर्मस्य तदात्मानं सृजाम्यहम् ॥४-७॥

परित्राणाय साधूनां विनाशाय च दुष्कृताम् ।

धर्मसंस्थापनार्थाय सम्भवामि युगे युगे ॥४-८॥

Yada yada hi dharmasya glanirbhavati bharata

Abhythanamadharmasya tadatmanam srijamyaham

Paritranaya sadhunang vinashay cha dushkritam
Dharmasangsthapanarthay sambhabami yuge yuge 

Here Krishna is telling Arjuna that whenever there is decay of "dharma" (a concept that is not easily translated in English in fact, but more commonly translated as "one's duty" or "righteousness") in the world he will be born again to restore righteousness and destroy evil.

Some people take this literally i.e. through reincarnation Krishna will be born again in a human form and take these actions. However, I think that is not what this means. Krishna is not a person here but he is knowledge, ideas. What he is saying here is that whenever evil ideas start taking over the world, good ideas will emerge to counter them. And I think this is true. I think there is some tendency for "good ideas" to emerge over and over again. "Bad/evil ideas" do prevail from time to time, sometimes for a long time, but so do "good ideas". And I don't think it is just a matter of evolution of ideas ("meme evolution") but rather there is some propensity for "good ideas" (which are closer to objective morality than bad ideas) to emerge. It is almost like objective morality seeds them in the brains of humans from time to time (like objective mathematics is seeded in the brains of some mathematicians). 

And that is "Krishna" being born over and over again to destroy evil in the world.