What is PFI and why did NIA and ED raid their offices all over India?

The National Investigation Agency launched a massive nationwide search operation in connection with anti-terror activities. The operation, being described as the “largest ever”, involves searches across 10 states, and has so far led to the arrest of as many as 100 people linked to the Popular Front of India (PFI). What is PFI and why did NIA and ED raid their offices all over India?