I think it's nowhere well explained in any of the widgets' conf **what preselect is**.
The explanations here in API doc are useful only for people who know the feature already.
I'd write a short explanation for each of the widgets.

In case of selectbox sth like:

When preselect is enabled (`always`), the first option on the list matching the input text is highlighted, so it can be selected upon hitting ENTER from keyboard. Otherwise (`none`), the user has to use ARROW_DOWN key first to select the match.