Mexico's central bank. It sets the policy rate (the one that moves every other rate), regulates payment systems such as SPEI, and publishes the FIX exchange rate and the average CAT (Costo Anual Total, the all-in annual cost figure) jointly with CONDUSEF. When Banxico raises the policy rate, loan rates climb a few weeks later; when it cuts, new mortgages adjust quickly.
Banxico sets the reference rate, and almost everything else hangs off it: the TIIE, your card's CAT, the yield on your Cetes. When it raises the rate to curb inflation, new credit gets pricier with a few months' lag; when it cuts, it takes about as long to ease.
For you as a user, the practical part is timing: if Banxico is in a cutting cycle and your loan is variable-rate, your payment may ease; if it's fixed, you won't notice. Decisions are announced on scheduled dates through the year — if you're about to take a variable mortgage, it's worth checking where the cycle points before signing.