What is Employee Retention Credit and how does a business owner claim this tax refund?

ERC is a stimulus program designed to help those businesses that were able to retain their employees during the Covid-19 pandemic.
Established by the CARES Act, it is a refundable tax credit – a grant, not a loan – that you can claim for your business. 
Business owners may qualify up to $26,000 per W2 employee or businesses that have availed of PPP, may still qualify up to $10,000 per W2 employee.