It is simply what it says.
A rate and term refinance is a process in which the homeowner plans to refinance his current mortgage with an intent to either change the rate, change the tenure, or, both.
For example, if you are paying 3.25% on a 15-year mortgage, and with the new refinance you are getting 2.125% with the 15 years mortgage, this would be a rate change.
If your new refinance rate is 2.125% on a 10-year mortgage, this means you are changing both rate and term.
Interest rates depend on the current mortgage market and your eligibility. However, the mortgage term is something that you can choose.
If you want to make sure you need to pay your mortgage quicker, and you have a good enough income, you can choose a 10 or 15 years mortgage while doing a rate and term refinance.
If you want to keep your monthly payments low and manage your monthly expenses, you can choose a 30-year mortgage during the rate and term refinance.
The thing to remember is that if you choose to go for higher tenure, the interest rate that you would get on your rate and term refinance would be higher.
For example, the interest rate on a 15-year rate and term refinance would be lower than a 30-year one.