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.
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.
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