Hi Ranjan!

JEE Mains is the examination which is conducted first, followed by JEE Advanced.

For example, this year, the JEE-Mains examination was held in two phases during 23rd to 29th July.

The students selected through JEE-Mains will get an opportunity to sit for JEE-advanced examination which is slated to be conducted on 28th August, 2022. The selections for various Indian Institute of Technology (IITs) is based on this examination.

I hope this helps! Best of luck!