You are storing a "pointer" to the cvar in a variable. This way, when you reference the cvar again later, you are not making AMX Mod X search through the giant list of cvars a second time. Rather than saying to AMX "hey can you check the value of that cvar again? I forgot where it is, so you'll have to look through the list again for the right cvar" you are instead saying "Hey, can you check the value of that cvar again? It was the 321st cvar last time we checked." or something similar.
https://forums.alliedmods.net/showthread.php?t=215635
The include is showing you what is required in the native call. For example, that particular native requires you to specify a Ham function, an entity on which the function is being executed, followed by any number of parameters of any type.
http://www.amxmodx.org/funcwiki.php?go=module&id=24
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