Based on User Profile or Account Settings automatic search in Forum and in Tickets could be enabled. Search box will be filled with the keyword entered in Subject, Description or Comment as soon as you type in respective field. Forum Topics and Tickets results are displayed below the search box. For Ticket search an additional option to limit search in tickets with matching Category could be enabled.
Note that this will work only for the respective control if the configuration is enabled in Knowledge base search setting in User Profile (or in Account Settings if the user profile has not been updated).