STONEWALL is a unique way of building transactions that increases the deniability of the link between sender and recipient of a transaction. A STONEWALL is designed to improve your privacy on the blockchain by introducing a large element of doubt and uncertainty into the datasets of blockchain analysis platforms. STONEWALL works using the Boltzmann script, which you can read more about here.
STONEWALL is enabled by default in Samourai wherever possible and there is no extra charge for using it. STONEWALL will not always be possible for every transaction. If you want to send a transaction without STONEWALL you can simply toggle STONEWALL off via the toggle switch on the Send screen.