The aptitude test in RRB NTPC applies to specific posts like Station Master and Traffic Assistant after CBT-2.
It is a computer-based test checking cognitive skills relevant to the post.
You must score a minimum qualifying mark in the aptitude test to stay in the merit list.
The final merit combines CBT-2 and aptitude test scores in the ratio given in the notification.
Check the official RRB notification to see if your post requires it.