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.

862. That 'it happened at the end of forty days' means the duration of the former state, and the beginning of the one that followed, is clear from the meaning of 'forty', see 730, where, the subject being temptation, the phrase 'forty days and forty nights' was used, which meant the duration of temptation. Here, since the subject is the state following temptation, 'forty days' is mentioned but not forty nights. The reason is that charity now starts to appear, which in the Word is compared to the day and is called the day. Faith however which precedes but has not yet been so joined to charity is compared to the night and is called the night, as in Genesis 1:16, and elsewhere in the Word. Faith is also called 'the night' in the Word because it receives its light from charity, just as the moon does from the sun. Faith is therefore also compared to the moon and is called the moon; and love or charity is compared to the sun and is called the sun. 'Forty days', or the duration of time meant by them, refers both to the things that precede and to those that follow; hence the statement 'at the end of forty days', which accordingly means both the duration of the previous state, and the beginning of the one being described now. This then begins the description of the second state following temptation of the member of this Church.