The search slot in RelBib is provided with an autocomplete function: Depending on the selection of the search aspect, the beginning of a word is completed to person names or subject headings in a meaningful way. Names of persons or subject headings that begin with the same letters appear in a list for selection. When searching via "All fields" or "Title", a list with titles is offered in which the searched word / part of the searched word appears. You can then select one of the suggested terms or titles and start the search by clicking on the red button.
You can find out more about “autocomplete” on Wikipedia.