Threshold and adjusted income are terms relevant for the tapered annual allowance.
In order for the tapered annual allowance to kick in (whereby annual allowance is reduced by £1 for every £2 above the threshold down to a minimum of £4,000), both threshold and adjusted income need to be above £200,000 and £240,000, respectively, as of 2020/21. Threshold income is generally just your regular earnings whereas adjusted income includes your pension contributions for the year.
See also our Tax Benefits of a Pension blog post.
