It refers to "incidental operating expenses" incurred in the productive investment of capital, which do not themselves add new value to output.
In Marx's social accounting, the faux frais are a component of constant capital, or alternately are funded by a fraction of the new surplus value.
These can include all kinds of things like bookkeeping, training, catering, cleaning & repairs, advertising, insurance, security services, bribes, taxes & levies etc.
In general, Marx seems to have regarded net insurance and tax payments from gross production income as part of surplus value.
In 2002, the US IRS tax-assessed capital costs of all US corporations with a positive net income included the following items: In the NIPAs, the Bureau of Economic Analysis adds $26.2 billion worth of expensing on meals and entertainment, oilwell bonus payments written off, adjustments for insurance carriers and savings and loan associations, amortization of intangible assets, and tax-exempt interest income.