Saidin is the male half of the True Source. That is, it is that part of the One Power which can be channeled by men.
Since just before the Breaking of The World, it has been tainted by the Dark One's influence, so that any male channeler goes mad, without a special bond to the Dark One such as the male Forsaken have.
Channeling with saidin cannot be detected by those who can channel saidar, and vice versa.