This option enables you to auto focus a editor instance. The value of this option should be a editor instance id. Editor instance ids are specified as "mce_editor_<index>" where index is a value starting from 0. So if there are 3 editor instances on a page these would have the following ids mce_editor_0, mce_editor_1 and mce_editor_2.
Example of usage of the auto_focus option:
tinyMCE.init({
...
auto_focus : "mce_editor_2"
});