The lecturer is saying
Initially the gradient is positive and "fairly constant", but for it drops to zero at the peak, it then becomes negative for a period before returning to zero.
figure_1
assume the initial part is the time frame between the starting point, shown above, and somewhere shown below
figure_2
the gradients at these 2 points is close, compared to the one shown below.
although, the gradients in figure_1 and figure_2 are obviously different, which seems not to be "fairly constant". The earlier the initial part, the better.
So, there a definition or convention about the initial part of a plot?