Employee engagement is the emotional commitment employees have to their work and organisation: whether they care about outcomes, discretionary effort and staying. It is usually measured through surveys and correlates with retention, productivity and customer outcomes.
The workplace contributes through connection. Office days that reliably put people with their team, in space that works, build the relationships engagement runs on. Office days spent on video calls at a random desk do the opposite, which is why coordination tools matter to an HR metric.