Clients mods are modifications to the client-side game. They can improve your gameplay, fix annoyances in the original game, or simply customize your game more to your preference.
The most popular Siege clientmod in yews is Sil's Client Mod (SMod). It is a lightweight community mod which updates JA with many important features (such as automatic map downloading), bugfixes (allowing unlimited custom maps) and optimizations. SMod is compatible with most server mods including base, JA+ and base_entranced.
There are also many mods for jka siege, which include visual mods, sound mods, etc. You can find them at Duo's JK3 Siege Mods - Check them out!