How to use $edit array in forms

Hello,

I'm trying to write a module, and I came on some form code that use the $edit array. I have not found any clear documentation on how to use this value. For now, my array is in the settings of the module and I use variable_get as the default value (which seems to work), but I have seen several bits of code with the edit array. Any link to a documentation ?

Thanks in advance...