STATA正交化处理以及残差问题

以下是我看到的一篇paper,红色部分是不明白的，我知道STATA处理正交的命令orthog,但应该对于以下这段话，应该如何处理呢？这里的残差（residual）是什么意思呀？谢谢！
To control for the effects of political traditions, we use two variables: a measure of the extent of democratic political systems in the country (Jaggers and Marshall, 2000) and a measure of the constraints on political actors in the system (Henisz, 2000). The two variables have significant correlation. To reduce the possibility of multicollinearity from inclusion of both in the same regression, we orthogonalize the democracy variable on political constraint and include only the residual value of political constraint.

我也很想知道