Basis is the amount of already-taxed money that was paid into the account.
For example, you generally have zero basis in a defined pension (because the employer funded it with tax-free money), and you generally have zero basis in a pre-tax IRA, 401k or other plan (because the money was deducted from your pay pre-tax, you never paid tax on it.)
You might have a basis if you made non-deductible contributions to a traditional IRA, but you need to have kept copies of your old form 8606s to prove it. Your basis in a Roth IRA is the amount of your original contributions because they were all made after tax.
