The Relevance bias function allows you to usefully fine-tune your search results towards finding matches in the title or within the text of items.
The Title bias option is useful for finding an item (for example, finding a case if you know part of the title/citation, such as the appellant name and year).
The Text bias option is useful if you are searching for information about a certain subject. It helps avoid matches in the title overly influencing the search results.
A useful real-world example is a search for 'HJ Iran' in the EIN case law database. If you set relevance bias to Title, you will find all cases with 'HJ Iran' in the title are ranked at the top of your results. If you set relevance bias to Text, the top results are cases that are about the case of HJ (Iran).
The default All (balanced) option biases your search somewhere between the Title and Text options. It provides a sensible balance for most searches.