Lets take eg. 3 - [3,2,2,4,1,4], D = 3. Now lets look at the algorithm steps.

Determine maximum weight - Think of a maximum weight ship can carry. This is the case if it has to carry all packages in a single day. It comes out to be total of all elements=16.
Determine minimum weight - Think of a minimum weight ship can carry - This is the case if it has to carry single package everyday. So minimum weight comes out to be maximum of all elements=4.
Start binary search for days - Now, we start the binary search. In weights = [3,2,2,4,1,4] our search space is from low = 4 to high = 16 1.
1. Iteration 1 - low = 4, high = 16, mid = 10, it takes us daysNeeded = 2 days. But days needed mid is more than D days, hence we reduce our capacity. So, high = mid - 1 = 9
2. Iteration 2 - low = 4, high = 9 ⇨ mid = 6. Now, in this case it does take 3 days, which is what we wanted. So, why not reduce the capacity further.