The price of money, quoted as an annual percentage. A 28% annual rate on 50,000 pesos over 24 months doesn't mean paying 14,000 pesos a year — the calculation shrinks as you amortize the principal. The rate on its own misleads: to compare offers, use the CAT (Costo Anual Total — Mexico's all-in annual cost figure), which does include fees.
Watch the gap between nominal and effective rate. A «3% monthly» sounds low, but annualized with compounding it climbs above 42%. Short-term SOFOMs often quote per week or per month precisely because the monthly figure looks small.
In Mexico almost all consumer credit is fixed-rate in pesos. Variable shows up mainly in TIIE-indexed mortgages and some business lines. If you're offered a «preferential rate», ask preferential to what and for how long: many promos fix the low rate for three or six months, then jump to the rate-sheet number.