Substantial completion is the point at which the project — or an agreed portion — is finished enough that the owner can occupy and use it for its intended purpose, even if minor punch-list items remain. It's a legally significant milestone, typically certified by the architect.
A lot hinges on this date: it usually stops the liquidated-damages clock, starts warranty periods, shifts responsibility for utilities and insurance to the owner, and triggers the release of most retainage. Reaching it is the finish line most of the schedule is built to hit; final completion (punch closed, closeout submitted) follows.