What is SAT?

Firstly lest see what SAT is. Best thing is to red about it in whitepaper:

SAT is the main chain project developed by SmartX’s community-centred team. It is a decentralized, minable BlockDAG project with a “transaction-based DAG” structure that can achieve limitless performance expansion, which is essentially equivalent to a traditional blockchain system that can achieve limitless horizontal expansion. SmartX’s platform will allow blockchain technology to be seamlessly adopted in use
cases such as private communication, in-game payment as well as in products for example, coupons and points given by shops and merchants. It links service providers and users around the world and uses social entertainment as a gateway to building a trustworthy and secure social entertainment ecosystem. SmartX will be an integrated and multi-functional platform for cashflow, dataflow as well as “value-flow”. In martX’s trust-based structure, intrinsic values of individuals and entities will be delivered through the network to form an ecosystem where both tangible and intangible values can be properly recognized. Such an ecosystem will greatly improve the efficiency of society as a whole.

What I need to do to get some free SAT?

You just need to go here enter your ETH address and claim your credits. Depending on how much credit you will claim each week you will get a corresponding amount of SAT then.
A lot of coins was distribute at beginning through faucets. Let just look at Bitcoin.