A recast applies a lump-sum principal payment and re-amortizes your loan at the same rate and term, lowering the monthly payment. Model it by reducing the loan amount and comparing payments.
You pay a lump sum toward principal and the lender recalculates your payment over the remaining term at the same rate — lowering the payment without refinancing.
A recast keeps your rate and has low fees but does not change your rate or term. Refinancing can change both but has closing costs. Compare with the refinance calculator.