Joint Entrance Examination – Advanced (JEE-Advanced), formerly the Indian Institutes of Technology-Joint Entrance Examination (IIT-JEE) is an annual engineering college entrance examination in India. It is conducted by one of the seven zonal IITs (IIT Roorkee, IIT Kharagpur, IIT Delhi, IIT Kanpur, IIT Bombay, IIT Madras, and IIT Guwhati ) under guidance of the Joint Admission Board(JAB). It is used as the sole admission test by the 23  (IITs). Other universities like the (IISERs), and the premier (IISc) also use the score obtained in JEE Advanced as the basis for admission. Any student who takes admission to IITs cannot appear for the JEE-Advanced exam in the next year, but the same is not the case with IISc, IISER, RGIPT and other institutes as these institutes only use JEE Advanced score for admission.

The examination is organised each year by one of the various IITs, on a round robin rotation pattern. It has a very low admission rate (about 9,369 in 479,651 in 2012;which is around 1.95%)The current admission rate in 2017 is around 0.92% in IITs (about 11,000 out of 1,200,000 who applied for JEE MAIN .It is recognised as one of the toughest examinations in the world and is one of the most difficult examinations of India to crack.

No thats not the case
All qualified students for mains can write jee advanced exam and then if you clear jee advanced that also doesnt sure you admission as your AIR must be in 10000 to get an IIT. The result stating "you have qualified jee advanced" means that you are among those people who have cleared the cutoff. If you have rank in top 10000 then you can get admission in IIT.