@JustTakeTheFirstStep pxGrid is a secure communication protocol, when configured ISE will share context information (User/IP binding, SGTs (Security Group Tag), SGT/IP bindings, endpoint profile information, threat data etc) with the FMC and other subscribed systems such as Stealthwatch, DNAC, WSA and other 3rd party solutions.
In regard to ISE/FMC integration, with this information learnt from pxgrid on the FMC, you will now have a username associated to the IP address. You can then create rules based on username or AD group (with AD realm) instead of just the IP address and also provide more useful information when looking at the firewall logs. You will also have the ability for the FMC to instruct ISE to quarantine the user/endpoint at source (the connected switchport) based on a firewall event (malicious activity).
https://www.ciscopress.com/articles/article.asp?p=2963461
