Mixin (aka Ring Size) is the degree of obfuscation applied to a sent transaction.
When a transaction is being formed by the wallet software, the user can select the desired mixin level, which adds additional signatures from recent similarly-sized transaction outputs to the current transaction being prepared. This is the purpose of the Ring Signature system, as used by Safex and other privacy coins using the CryptoNote blockchain protocol.
Safex Wallets have a default ring size of 7, which means 7 additional transaction signatures are mixed into the transaction, and any third party looking at the transaction has no way of telling which of the 8 signatures actually relates to the real transaction being made.
These extra signatures add additional data to the transaction size, which results in a more expensive transaction fee.
When preparing to send a transaction in the wallet, users can select different mixin values, according to their personal preference. The cli wallet allows the user to enter any value; however, GUI wallets normally provide a dropdown selection from 1 to 7.
Read more about Ring Signature.