alt Hacker NewsIt's a standardized mean difference, which I believe can roughly be interpreted as: "treated groups had 0.67 stddev lower depression score than control groups."
That's a pretty substantial improvement - consider someone who's more depressed than 75% of the population becoming completely average. (Because the 75th percentile is about 0.67stddev above the median.)
You cannot say if this is a substantial change or not, because you need to know by how much the groups actually differ on average, i.e. you need the unstandardized effect size, expressed as a mean difference in the scale sum scores, or as an actual percentage of symptoms reduced, or etc. In general, there are monstrous issues with standardized mean differences, even setting aside the interpretability issues [1-3].
See also my response to GP.
[1] https://journals.plos.org/mentalhealth/article?id=10.1371/jo...
[2] https://bpspsychub.onlinelibrary.wiley.com/doi/abs/10.1348/0...).
[3] https://www.tandfonline.com/doi/abs/10.1080/00031305.2018.15...