Harish,
An agent is a small part of the Orchestrator Engine that you install on remote systems to allow parts of PAM processes to run on that particular server. You can get a high level overview of all Process Automation components here:
https://docops.ca.com/display/CATPA43/System+Components
A simple example would be if you need to read, or write a file onto a specific, remote, server. You could install an agent on that remote server to allow access to that local file system.