Occasionally you may not have time to vote but still, want to receive maximum exposure boost.
The Fill allows you to skip voting by automatically filling your 'Exposure Boost' meter in all of the challenges you are participating in.
It does not keep the 'Exposure Boost' up for the entire challenge.
Note - The Fill does not automatically vote for random photos, it simply fills up your exposure meter.
Once you use a Fill the exposure meter will start to decrease the same way as if you voted.