Mmmhmmm is an expression usually uttered to indicate agreeance.
Instead of saying yes, one may use mmmhmmm.
As told by Robert Thompson to NPR, an art history professor at Yale University, the phrase was used among slaves so as to not stir any trouble. Then the phrase started being used among African-Americans, then whites. Thompson argues that whites use to use “yay” and “yes” rather than mmmhmmm.