We've heard it called many things...
Post hole foam
Pole setting foam
Expanding post hole filler

All of the above are acceptable answers.

Whatever you call it, it's basically an expanding post hole foam or pole setting foam, used to set posts or poles that takes the place of concrete. It is lightweight, requires no water or intense mixing, and sets in just minutes. It is most commonly used to; set a fence post, set a mailbox post, set a utility pole, set a light pole, set a telephone pole, or even to set a deck or porch footing post. One thing is for sure, it is a better alternative to concrete. You can use this expanding post hole foam without breaking your back or taking up all of your time. You can set a post fast and easy with little worry of transporting or installing.

http://www.foamapost.com/

Lightweight – 3lbs per post hole

Easy to lift!

Heavy – 40lbs per post hole

Backbreaking!

Small package – Less room on the shelf or vehicle, more room for other products.

Bulky package – takes up more room on the shelf or vehicle, less room for other products

Affordable to ship – Over 400 packages fit on a single pallet!

Expensive to ship – Only about 50 bags fit on a pallet

Installation

Does not require water

Requires water

Use in all climates - Can be used in below freezing temperatures

No use in colder climates - Cannot be used in below freezing temperatures

Sets quickly – Can be installed by one installer in minutes, braces and pickets can be installed same day

Sets slowly – Requires two installers, someone has to return next day to install braces and pickets

Protects the post – Waterproof, Prevents degrading and rot of the post

Deteriorates the post – Porous aggregate, Holds moisture

Temporary Installations – removes easily with little effort (Example: Real Estate Signs )

Permanent – Very heavy and hard to remove