The string.encode("us-ascii") method converts all the characters in the string to United States ASCII 7-bit values.
US-ASCII is essentially plain text with 128 characters total. That encoding was common on United States computers in the 1970s-1990s.
The reason you're seeing it now is probably because you're using email. The email protocol requires US-ASCII encoding for strings.
Ruby 1.8.7 doesn't have string encoding methods built-in because Ruby 1.8.7 stores strings as bytes, not encoded characters.
To convert in Ruby 1.8.7, see the Iconv library:
http://ruby-doc.org/stdlib-1.8.7/libdoc/iconv/rdoc/Iconv.html
Also see the conversion iconv code sample in this answer:
String.force_encoding() in Ruby 1.8.7 (or Rails 2.x)