Launch daemons are processes that are launched upon system startup (or at a later time) and run in the background.
Disabling daemons allows you to break core system functions (such as OTA updates, Game Center, error logging, etc.).
As such, please be aware that software relying on the daemons you disable may stop functioning properly. While disabling some processes that run at load can somewhat improve the performance of very old devices (iPhone 3GS and earlier), it is still not recommended to do so unless you really know what you're doing.
Also note that disabling daemons has literally no impact on the performance of newer devices.