As you probably already know from Let TranslateIt! suggest, TranslateIt! is a pretty smart application: As soon as you start typing a query, it starts scanning all active dictionaries for suggestions. Though this feature is very helpful, due to its resource-intensiveness, it may decrease TranslateIt! performance and be quite annoying.
That's why TranslateIt! allows users to fine-tune its suggesting behavior.
The chosen number (except 0) tells TranslateIt! after how many characters typed in the search box it should start offering its suggestions. The less is this number, the sooner TranslateIt! will start suggesting, giving you more suggestions, but requiring more time for the process. Therefore it is recommended to choose 3 or 4 on slower computers.
Users are also allowed to fully disable automatic suggesting by choosing the "0" option. (In this case, if needed, suggestions should be requested manually. To find out how, see Let TranslateIt! suggest.)