A mortgage stress rate is the interest rate used to calculate the rental cover requirement for a buy-to-let mortgage.
The stress rate used varies between lenders, product and the borrowers situation. For example it will often be different for a portfolio landlord compared to a first-time landlord.
Currently (early 2019) typical rates in use are around 5%.
You can use the free PaTMa buy-to-let yield calculator to calculate rental cover with different stress rates.