Customer engagement is how much people actively do something with a brand rather than passively receive it. On a screen network it is a touch, a scan, a dwell longer than a glance; in video it is a choice made rather than a view counted.
It is worth measuring because attention is the scarce resource in a physical space. Someone who interacted has demonstrably chosen to, which is a far stronger signal than being nearby when something played.
The caveat is that engagement is the most gameable metric in the category. Anything can raise it (a prize, a game, a question people answer reflexively) without moving a single business outcome. A campaign optimised for engagement reliably produces interactions with people who were never going to buy.
The version worth reporting connects to something identifiable downstream: a code redeemed, an account created, a visit that turned into a sale. Engagement is a leading indicator when tied to an outcome, and a vanity number when it stands alone.