Autocorrelation means that a text is compared to shifted copies of the same text. The characters from both texts, that match each other in such a comparison, are determined.
Thus, it is for example possible to crack the Vigenère cipher. Periodically repeating maximum values of matching characters indicate the key length of the Vigenère cipher.
An autocorrelation analysis of a sufficiently long message encoded with the Vigenère cipher and a key length of eight characters would roughly yield the following result:
Fig. 1: Autocorrelation1
The autocorrelation analysis is more efficient and clearer than the Friedman or Kasiski test. The analysis is versatile and can also be used for example in signal processing.1 A frequency analysis is a complex calculation and thus it is advisable to use locally installed tools such as CrypTool for large texts.
1 Brätz, Marcel: "Kryptographiespielplatz", http://www.kryptographiespielplatz.de