Yes. Typically, these annuities are funded with money from 401(k)s or other tax-deferred retirement accounts, such as IRAs. When you receive payments from a qualified annuity, those payments are fully taxable as income. That's because no taxes have been paid on that money.
Box 2a, Taxable Amount: This is the portion of pre-tax money of your Gross Distribution (in Box 1). Any post-tax money distributed would be represented in Box 5.