Andolan is a delicate, slow oscillation: the note is not held perfectly still but gently swung, swaying a little and back, like a slow vibration. It is subtler than a forceful gamak and more localised than a gliding meend.
In some raags, andolan on a particular note is not decoration but identity. The oscillated komal Ga and komal Dha of Darbari are textbook examples — without that precise sway the raag is unrecognisable. Raags such as Miyan ki Todi and Bhairav likewise depend on andolan.
Mastering andolan means controlling exactly how wide, how slow and through which shrutis the note sways.