How are businesses taxed twice?

I hear business owners say this a lot. They say they are taxed twice. What exactly do they mean? The last person who said it to me was referring to having to pay his total social security obligation, rather than having an employer pay half. But, I assume that’s not all that small business owners are talking about. It seems to me business owners have a lot of write-off options that employees don’t have. Maybe it balances out?