whenever we report (opinion on financial info) or certify (accuracy of financial info), such assignments are called attest functions. As such reports / certificates are evidence that financial info are reliable or not and increases confidence of users. literal meaning of attest means something which provides evidence.
such reports or certificates are generally issued as per legal requirement or sometimes client may voluntarily ask for it.  
in short reporting and certification assignments are attest functions.
Every Book has starting portion which gives purpose / overview / precautions while reading book, this portion is called preface. SAs also has similar preface. Matters discussed in lecture intro to SAs is picked from preface to SAs