FICA is the payroll tax that funds social security and medicare. The name is the Federal Insurance Contributions Act.
Paid twice on the same wage
An employee has a share withheld from their gross pay. The business then pays a matching share of its own, which is a cost to the business rather than a deduction from anybody.
So a wage costs the business the gross figure plus its half of FICA, before anything else.
Self-employment
Someone working for themselves pays both halves, which is what self-employment tax is. A contractor's rate has to cover this where an employee's wage does not — see employee or contractor.
Why it shapes owner pay
FICA applies to wages and not to distributions. That gap is precisely why reasonable compensation rules exist.