One loan over several properties
A blanket loan for multiple rental properties is a single mortgage secured by more than one property at once. Instead of five loans, five payments and five closings there is one of each, and all five properties stand behind the same debt.
It also gets called a portfolio loan or a blanket mortgage. The mechanic that matters is cross-collateralisation: the lender holds a lien on every property in the pool, so the properties stop being independent of each other.