Motivation and habit
Zeigarnik effect
Unfinished tasks stay on people's minds, so a task that's started and visibly incomplete tends to pull them back to finish it.
Something interrupted keeps nagging until it's done. A half-finished task holds attention in a way a finished one doesn't, and people are more likely to go back to it.
Products use this with setup checklists, unfinished drafts and 'pick up where you left off'. Breaking a long task into visible steps helps too: each step finished shows how little is left.
In practice
An online course that ends a lesson partway into the next topic, with the next video queued, tends to bring more people back than one that ends every lesson neatly.
Common mistake
Leaving people with an endless list of unfinished things. Nagging about tasks that don't matter to them turns a helpful pull into guilt.
Where it comes from
Named after Bluma Zeigarnik, who in 1927 reported that people remembered interrupted tasks better than finished ones, after her teacher Kurt Lewin noticed waiters remembering only unpaid orders. The memory effect has been hard to repeat reliably; the urge to resume an interrupted task, studied by Maria Ovsiankina in 1928, holds up better.