[StringName]s are immutable strings designed for general-purpose representation of unique names (also called "string interning"). Two [StringName]s with the same value are the same object. Comparing them is extremely fast compared to regular [String]s.
You will usually just pass a [String] to methods expecting a [StringName] and it will be automatically converted, but you may occasionally want to construct a [StringName] ahead of time with the [StringName] constructor or, in GDScript, the literal syntax [code]@"example"[/code].
See also [NodePath], which is a similar concept specifically designed to store pre-parsed scene tree paths.