A direct hit search searches for your particular mark, within the class of goods and services within which your mark is to be used. It's a narrower search that identifies the most challenging previously-registered marks for your trademark registration.
When someone files a trademark, if the trademark is not an English word, then the Examiner will attribute one or more "pseudomarks" to it, and these are English language words that are similar or evoked by your trademark. The pseudomarks are associated with the trademark in the Register. For example, if the trademark is Harry Potter, pseudomarks might be "hairy" and "pot".
When a trademark is filed, it is also associated with certain goods and services that the mark is being used with. These need to be specified at the time of filing, and are categorized by the Trademark Examiner into one or more classes (there are 45 international classes that are used, 1 - 30 for goods, and 31 - 45 for services).
A direct hit search searches the trademark itself, along with the pseudomarks, or English equivalents, within the particular classes. These are the search results that are most likely to stand in your way. However, an expanded search will look at other trademarks that may cause problems for you, including those in associated classes, not just the identical class, as well as marks that sound like or evoke similar ideas to your trademark. Finally, unregistered marks may stand in the way of your registration, but cannot be found in the Register.