Question on add_hook()

I'm trying to understand hooks -- would someone please tell me what the number 1 represents here:

add_hook("ClientAdd",1,"create_forum_account");

I'm sure it's simple, but I'm not finding it in the documentation.

Thanks

"ClientAdd" is the action point, this is when the assigned function will run (Client Signed Up, Order Placed, etc)

"1" is the priority number where all action hook functions will sort and run Ascending, you can set it to 0 so it will work before any function set with 1

"create_forum_account" the function name to run

Great, thank you!