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It's a metamod module. It's something you install like AMX Mod X, for example.
If not done already, install metamod ;
Download http_resources_manager-files.zip and extract the content in your addons/ directory ;
Open addons/metamod/plugins.ini file, add the following line and save :
If windows server : win32 addons/http_resources_manager/dlls/http_resources_manager_mm.dll
If linux server : linux addons/http_resources_manager/dlls/http_resources_manager_mm_i386.so
If your server is behind a router, you can explicit the internet IP by adding the following cvar in your server.cfg : ip "x.x.x.x". 
You must provide it otherwise your players won't be able to connect to your server ;
Configure as you wish ;
Restart your server ;
Check if all is good using the server commands : rm status and rm config.