It's an employer benefit that reduces your employer's National Insurance liability.
Employment Allowance is £10,500 per year.
Each time you run your payroll you pay less employers’ Class 1 National Insurance. This continues until you've used the full allowance for the tax year or until the tax year ends.
CAUTION:
Eligibility changes from one year to another, so you must apply again at the start of each tax year
When am I not eligible?
Over 99% of micro-businesses and 93% of small businesses can claim the allowance.
Regardless of the size of your company, you can’t claim the employment allowance if:
You’re the director and the only employee that’s paid above the secondary threshold
You employ someone for personal, household or domestic work. Unless they’re a care or support worker
You’re a public body or business doing more than half of your work in the public sector. Excluding charities
You’re a service company working. You're under IR35 rules, and your only income is the earnings of the intermediary
For more information about Employment Allowance, read the HMRC article at gov.uk.