When was your company founded? This is the formal date of incorporation, signing the partnership agreement, or registering the business, depending on what form of legal entity your company is. This also might be asked in the form of number of years in business, and this is the number of years since the company became a legal entity.
They may be asking this just for their records, but likely they are asking so that they know if your company has been in business for a while. The likelihood that a business will continue to operate increases as the company gets older and grows. Many businesses fail in the first 3 years.
They are expecting a simple answer of the date of establishment of the business or the number of years in business, depending on the exact question.