I think it's a 1% explanatory effect of the total variance. That means that 1% of the variance in the population (not the size of the effect) is explainable by this factor.

(Also, the base rate of depression is far more than 1%, see e.g. http://tech.mit.edu/V120/N3/dep1.3n.html - it's around 17% for just a major depressive episode.)