The second is the replacement characters. Tr returns the number of characters replaced. Both the SearchList and ReplaceList can be a list of characters or a range of characters. If ReplaceList is shorter than SearchList, the last character of ReplaceList is replicated as necessary. This tells tr to delete every instance of characters in the first argument.
Regular expressions are a powerful way of identifying patterns within a string.
Here are just a few of many. S takes the first argument as the pattern you are wanting to match. The second is the replacement characters. S returns the number of characters replaced. This allows you to match an instance of a string or character within another for use in commands involving truth such as if statements.