A group combines members of a dimension into bigger buckets without writing a calculation. Right-click a dimension in the Data pane and choose Create > Group.... Tick the members that belong together, give the group a name and click Group. Tableau adds a new field, for example Ship Mode (group), with a paperclip icon.
Members you don't group keep their own names. Tick Include 'Other' to sweep them all into one bucket called Other instead. That turns a long list into a short comparison: your groups against everything else.
Groups are fixed lists. If a new member appears in the data, it lands in Other (or on its own), not in one of your groups. When membership should follow a rule, like "the top 20 customers", use a set instead (later in this unit).