It's a landmark for a young child to count to 100; it sort of covers all the "ordinary" numbers. One hundred is obviously significant for other groupings: 100 cents is a dollar; 100 yards is a touchdown; 100 years is a century, and the landmark for a very long life. It makes sense, then, that in the Bible, 100 represents fullness or a state of completion, or in some instances simply "much." For instance, people marvel that Abraham had Isaac when he was 100 years old; the number represents the point at which the Lord, when growing up as Jesus, united the human elements of himself with the divine elements and in a sense became "complete.

884. 'And behold, in its mouth an olive leaf plucked off' means a small measure of the truth of faith; 'leaf' meaning truth, 'olive' good that stems from charity, 'plucked off' the fact that the truth of faith derives from that charity, and 'in its mouth' the fact that it was made visible. This is clear from the meaning of 'olive' and is evident from the very words themselves. That a tiny quantity is meant is clear from the fact that it was a leaf and no more.