Hello, I am trying to understand multiple linear regression models. I was reading something in the textbook, and I'm a little confused. How would I approach this?.

In a typical multiple linear regression model where x1 and x2 are two non-random regressors, the expected value of the response variable y given x1 and x2 is denoted by E(y | x1, x2). Build the model for the scenario where one of the regressors may change the value of E() | x1, x2) differentially as the value of the other regressor changes. How can such a potential change be assessed statistically