This achievement is for watching 100 video clips.
For me the clips were not counting as watched whilst fullscreen (as in pressing B in the app to remove the sports guide) so I highly suggest watching all your clips either snapped or with the sports guide open. This does unfortunately mean you cannot fast forward through them
Make sure you watch the relevant clips for other achievements which will give you at least 38 clips already watched:
The Fan Favorite achievement in ESPN worth 0 pointsUse More Sports to watch videos from 3 other sports The Hot Streak achievement in ESPN worth 0 pointsWatch at least 5 clips from your Favorites channel The Around the Network achievement in ESPN worth 0 pointsWatch 10 clips from each of the ESPN editorial channels
You can watch the same clip over and over so Channel 05 Must See Moments which is full of minute to 30seconds clips is aptly named to be must see for this achievement.