The Remi (Les RPM de Remi) repository provides the latest version of various software packages related to php and mysql for redhat based linux distros like centos, fedora and RHEL. It provides php, mysql, pecl packages, pear packages any many other open source/free php applications and libraries and many other php related packages. So its designed to assist in setting up apache+php based web servers with various kinds of open source applications. The default centos/fedora distros do no have the latest versions of these packages so repositories like Remi do the job.