When an account offers a tiered interest rate this means the interest rate you receive depends on the amount/balance in your account. When the balance goes above a specified level the interest rate increases on the whole balance. Tax-free indicates that interest earned is exempt from UK income tax.
This is the period during which you are ‘locked in’ to your mortgage deal. You’ll have to pay an early repayment charge if you leave your mortgage during this period. Avoid mortgages that tie you in after your introductory rate has ended.