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An Adjustable Rate Mortgage (ARM) usually starts with a low "teaser rate" for 3, 5, or 7 years. Once that period ends, the interest rate "resets" to match current market conditions. If rates have gone up significantly (as they did in 2022-2023), your monthly payment can spike overnight, sometimes increasing by hundreds of dollars.
If you have an ARM and you know a reset is coming, the best strategy is often to refinance into a Fixed-Rate Mortgage before the adjustment date. This locks in your payment and removes the uncertainty of future market fluctuations.