What is an alias in Tableau?

An alias is an alternative display name for a member of a dimension — the values, not the field.

Right-click the dimension → Aliases, and you can rename each member. `NY` becomes `New York`, `1` becomes `Active`. The underlying data is untouched, so filters and joins still work on the real values.

Aliases apply to discrete dimensions only, not to measures or continuous fields.

The related-but-different thing is renaming the field itself, which you do by double-clicking it in the Data pane. Interviewers occasionally ask both to see whether you distinguish the field from its members.