In regular expressions, a period can stand for any character. Normally, the newline character is not included, but most tools have optional switches to force inclusion of the newline character also. Using a period in a pattern is a way of requiring that "something" occurs here, without having to decide what.
Users who are familiar with DOS command-line wildcards will know the question-mark as filling the role of "some character" in command masks. But in regular expressions, the question-mark has a different meaning, and the period is used as a wildcard.