What is it in reference to? A job, or a return, or what?
Like, “tax position” could be a position taken on a tax return, which basically just means what you decided to report/do. Say a person has a hobby business – they have a lot of expenses and a little bit of income. You ask them if they do it to make money or just to have fun; they say they want to have fun but like to make money while they're at it. Their expenses for the year are greater than their income. Do you report this as a Sch C small business, resulting in a loss on the face of their return reducing their taxable income, or do you rule it a hobby and only allow expenses to reduce income to zero? (I think that's how it works…I might be totally off-base, but it will at least work for the illustration. 😛 ) This is something that's not 100% clear, so you and your client have to take a position on it in choosing how to report it. So, if you decide to report it as a small business, then you took the tax position that it was a small business.
In reference to a job, it'd be a job/position primarily doing taxes – probably income taxes, though it could include sales and payroll taxes.
I can't think of any other uses off-hand, but let us know the context and we'll try to help explain it. 🙂