Your reminders — times you've added yourself, the same every day until you change them.
Smart reminders — suggested automatically from what's actually on that day. They can be different every day, and there are none at all on empty days.
Grouped reminders — when several smart reminders would land close together, they're merged into one, so you're not pinged too often.
If something that day is marked Important, Smart mode adds a separate reminder about 1.5 hours before each one. If nothing is marked important, you get one summary reminder instead, timed to when things on that day are typically scheduled (or added, if nothing has a set time).
If you haven't set any reminders of your own, Smart mode also adds one general summary reminder ahead of everything else — as if you'd set it up yourself.
Reminders are delivered as real push notifications when possible, even with the app closed. Where that isn't available on your device/browser, you'll instead get a reminder while the app happens to be open.
Smart times are recalculated each time you open the app, based on that day's events — open the app at least once a day to keep them accurate.
Smart reminders never fire before your Wake-up strategy time — if one would land earlier, it's pushed to right when you wake up instead. Do Not Disturb pauses every reminder, your own and Smart ones alike, until you turn it back off.
Smart reminders never fire before this time.