The adaptation of letterforms to the size they’ll be read at: sturdier shapes and looser spacing at small sizes, finer detail and tighter spacing at display sizes.
The variable font applies optical size automatically, so captions stay legible and headlines stay sharp.
Metal type had optical size by necessity: every size was cut separately.
Digital type lost it by default. Variable fonts brought it back through
the opsz axis. If your typeface has one, leave it on; captions set with
display proportions are why small text sometimes looks spindly.