Why mod_flvx ?
Mod_flvx was coded by Paul Querna which allows people to skip boring parts or see video ending without loading the whole file (saves a lot of bandwidth). The default apache does not support this feature so we need to install mod_flvx to enable it. Lighttpd comes with the mod_flv_streaming module inbuild and that why people go for apache + lighttpd (as backend to serve flv files) to boost up their video streaming.

Pre-request :
You need to have apache 2.x or 2.2.x version installed in your server (unfortunately it wont work with apache 1.x version), also with httpd-devel rpm which has the apxs and a pseudo-streaming compliant Flash player Flowplayer .

Installing mod_flvx :
Download the mod_flvx from http://people.apache.org/Epquerna/modules/mod_flvx.c in any of your src directory.
cd /usr/src
wget http://people.apache.org/%7Epquerna/modules/mod_flvx.c
which apxs (to check the location of apxs binary)
/usr/bin/apxs -cia mod_flvx.c
apxs will setup everything for you (make sure that no errors during the compilation process)
Open your httpd.conf file and add AddHandler flv-stream .flv (at the end of AddHandler lines)
Restart apache /etc/init.d/httpd restart