Yes the link seems to be broken for the moment
but I assume that's the good site...
So QoS means Quality of Service and is used to differentiate service. For example FTP is a service which takes the max bandwidth it can, so thanks QoS you can tell that FTP will be allowed to only use 50Kb/s instead of 512Kb/s... You can also do such things based on IP instead of ports
If HTB projects is still dead on sourceforge, have a look at CBQ (https://sourceforge.net/projects/cbqinit
)... it's the same philosophy. But it's based on another algorithm, less precise with rate and so less efficient with high rate