It's supposed to be "potting compound", which is a form of epoxy, and if the right one is chosen it's very hard to get it off with anything short of fuming nitric acid. Combined with the microwires trick it can be made extremely difficult to get at the PCB without triggering the tamper detection.

See FIPS 140: https://webcache.googleusercontent.com/search?q=cache:HERKv5... (google cache version because the US has decided to be a failed state for a few weeks again)