From a more practical point of view, the program to convert between different character sets is called
iconv. It can be used on the command line from the OS X (Mac) terminal, as well as in most Linux distributions. The most popular implementation is GNU libiconv, providing support for hundreds of different encodings, including MacRoman and the other Mac encodings (see below). It can be used like so:
iconv -f macroman -t utf-8 infile outfile
iconv can also convert to many specialty and legacy encodings, so it's a worthwhile element of your retrocomputing toolkit. Most programming languages provide similar functionality through an
iconv() function of some sort (e.g. php, C).
Also thought I'd add - MacRoman isn't the only Mac encoding, there was one for each region:
MAC MACINTOSH MACROMAN CSMACINTOSH
However, from an English-speaking perspective, MacRoman is the only one commonly encountered nowadays.