Branche here means 'industry'. It's a synonymous word in French with 'industrie', e.g. the motor industry: la branche automobile / l'industrie automobile
As such, the column refers to the industry average figure for each channels displayed on the table.
Remember that for each data on a case interview, we need to compare it to both historical figures as well as industry average figures in order to see if there is any peculiarity with the given number before commencing any deep dive analysis.
Hope it helps.