So I'm learning basic econometrics. I'm looking at a linear regression model on Stata where I have to qualitatively compare a restricted model, where $y_i = b_1 + b_2 x_i$ and $y_i = b_1 + b_2x_i + b_3z_i$ and where $z_i$ is a dummy variable. 

I want to ask, what does it mean if the coefficient on $x_i$ increases after the addition of the variable $z_i$?