Focus is a set of filter tools. When you have defined a Focus filter to your liking, using one or more of the tools, you can bookmark it and apply it to your collection with a single click.
The Focus tools vary according to how you are viewing your collection (Artist, Album, Track, Composer, Work). They are accessed from the button marked Focus with the lens shutter graphic to the top left, next to the Play All blue button.
You can apply multiple tools to a filter eg: Jazz genre between 1953 and 1957. The logic is AND between tools and OR within a tool.
Many of the Focus tools are self-explanatory, some of the interactive tools are described further below (using the Album group of tools).
The Filter box tool searches a text string. It is always literal and doesn’t accept wildcards.
The Added tool uses the Import Date in the metadata. When clicked/tapped there is a bottom menu for relational periods, or you can drag the calendar boxes to define a date or range of dates.
The Years tool, when clicked/tapped, enables a decade to be selected or a start and end year to be defined.
The Genre tool, when clicked/tapped, enables one or more genres to be added to a filter. Genres are default or as mapped by the user.
The Format tool contains options for file type, resolution etc.
The Inspector tool is a miscellaneous set of options which are often helpful when grooming your collection. It includes ability to filter by file location which can be very useful.
Edit: see @brian 's recent post in this thread for further reference on Focus, with screenshots.