PEAR stands for: PHP Extension and Application Repository. Pear is an extension library for PHP and should be available as a separate package. It is similar to CPAN for pearl. PEAR provides things like enhanced mail functions, graphic functions, etc. that Roundcube depends on to function. You should be able to download it from your distributions repository using the standard tools.
Here is a
link (ubuntu documentation) that discusses using PEAR after it has been installed. The functions in this document should be largely distribution agnostic (e.g. it uses sudo for root) and useful regardless of what your running on.
