Statuses

Each item has one of three statuses, shown on the right of its row:

To do — the default for new items.
In progress — work has started.
Done — the item is complete.

The checkbox and the status stay in sync: ticking an item sets it to Done, and unticking it sets it back to To do. Choosing Done from the status menu ticks the box.

Only Done counts towards progress and satisfies the workflow validators — In progress still counts as open.