• Publish now — goes out within a minute (text and image posts usually within seconds).
• Schedule — pick a date and time for that channel. Different channels can have different times.
• Add to queue — fills the channel's next free posting slot (Publishing → Queue). Paid plans.
• Schedule on the network — Facebook and YouTube can hold the post themselves and publish it at the time you set. Handy if you want it visible in the network's own scheduler.

Drag a post to another day on the Calendar to reschedule it; the time of day is kept.