In to the metamod website is explained very well -
http://metamod.org/ .
" Metamod is a plugin/DLL manager that sits between the Half-Life Engine and an HL Game mod, allowing the dynamic loading/unloading of mod-like DLL plugins to add functionality to the HL server or game mod."
So to run your mods you don't need metamod at all. But if you want to add something custom to the MOD you just put Metamod in it and download or write your custom plugins that you will add to metamod to load them and to put custom stuff on to the MOD. Actually you can write plugins and load them without Metamod but most of us now use it because it give us nice and easy futures.
Btw read the readme file that come with HPB bot and you will understand more.