100% is 'the whole' - it's everything you are talking about.

100% of the large square is the whole square.

100% of a cake is the whole cake.

100% of a sum of money is the total amount of money.

100% of a group of people is the whole group.

100% effort is the maximum effort you can make - so people who say, 'I'll give it 110%' don't really understand percentages!