The lecturer is saying
Initially the gradient is positive and
fairlyconstant, but for it drops to zero at the peak, it then becomes negative for a period before returning to zero.
Cambridge gives this explanation about
more than average, but less than very
which seems not to be the case here.
assume the initial part is the time frame between the starting point, shown above, and somewhere shown below
Technically, in the initial part, the gradient (the slop of the orange line) is not a constant. but it could be viewed as a constant, when compared to the one shown below.
If the meaning "more than average" is taken, where is the average? So, it seems that "fairly" mean "relatively" in this case, is my understanding right?