The other activities are the estimated amount of calories that you will burn in a day based upon your stated activity level (Moderate, Sedentary, etc). Any activities that you log will add to that.
My default for "other Activities" was 1800 calories burned a day, notwithstanding that my setting was a "Sedentary Lifestyle". I think it was too high was manually reduced it to 1600, though even that may be too high.
I found that I was always using about 500 calories/day more than I consumed at such a high setting for "Other Activities", though this link suggests that calorie Count have it about right with their default settings:
Maybe I got something wrong with my metabolism, I'm logging everything I eat but still usually have a big deficit in calories/day, I should be losing weight every day but it's remaining constant.