DD-WRT is a Linux-based firmware for wireless routers and wireless access points. It is compatible with several models of routers and access points, for example, the Linksys WRT54G series (including the WRT54GL and WRT54GS). DD-WRT is one of the third-party firmwares, which are designed to replace the original firmware on some commercial routers. Alternative firmware may offer features and functionality sets that differ from the original firmware it is replacing.
DD-WRT includes such features as support for the Kai network, daemon-based services, IPv6, Wireless Distribution System, RADIUS, advanced quality of service, radio output power control, overclocking capability, and software support for a Secure Digital card hardware modification.
Ok now that I have given you the definition of what DD-WRT our next step is to define the problem. A while back I spoke about a network that had issues with bandwidth usage. To put it simply, the users were streaming music and movies on their business network and the network was dying.
One method of handling that issue is to limit the bandwidth used by the ip address or host name.
Another method is to limit applications and ports.
DD-WRT is software that can upgrade your router (if it is compatible) so it can use qos for bandwidth control.
(Note: DD-WRT firmware does not supports Bandwidth limit to each users. Only the paid version of DD-WRT firmware supports it. Here is a link to a workaround that allows this option on the free version link)