Min == minimum.
So Activity is the lesser of time*14 and total posts.
Time is the number of two week periods in which you have made at least one post. The two week periods are determined from Unix time.
In your case, you have made posts in four of the two week periods since your registration (i.e. 4*14 = 56). At the end of the current two week period your Activity will increment by 14.
It has to be said that the Activity won't increment automatically. It will increment in the next period only when you'll have posted through that period's duration. So only the
time * 14 (fortnights) when you have posted are counted for.