As per https://www.stellar.org/developers/guides/concepts/multi-sig.html
Low security consists of "Transaction processing" (Charging a fee or updating the sequence number for the source account). Which operations from this list fall into the low security category?
Example: Payment operation will update the sequence of the source account. So does the payment operation fall under low threshold ?
Transaction processing is not refering to a group of operations and also is not a operation itself. It is kind of creating the transaction itself.
Let me explain it with an example:
Account A creates a payment transaction on behalf of B who sends some cash to C (the transaction is sourced from A, it contains one payment operation from B). Although A is a multisig account it needs only one signature because the act of crafting a transaction itself is low weight. Account A will be charged the XLM network fee and A will get its sequence number increased by one. (Obviously B will also need to sign the transaction cause it's his money)
The only low weight operations are AllowTrust and BumpSequence as stated in the docs.
