Hello,
These are basic questions, when you have the material availability confirmed quantity for components during the ATP check system turn the light to green.
I would suggest to read SAP help document on it.
Checking Collective Availability Online - Process Orders (PP-PI-POR) - SAP Library
In MDPV if you select the Check mode you have an option at bottom of screen to Reset Availability.
Best Regards,
R.Brahmankar