If you are surprised, you can't move or act on your turn and you can't take reactions until your first turn ends. Given the above description, we can almost consider 'surprised' to be a condition. Where it says 'doesn't notice a threat', is that 'any' threat, or 'all' threats? It's that final sentence of the first paragraph.

Most of this is completely straight forward except for one part. A member of a group can be surprised even if the other members aren't. If you're surprised, you can't move or take an action on your first turn of the combat, and you can't take a reaction until that turn ends. Any character or monster that doesn't notice a threat is surprised at the start of the encounter. Otherwise, the DM compares the Dexterity (Stealth) checks of anyone hiding with the passive Wisdom (Perception) score of each creature on the opposing side. If neither side tries to be stealthy, they automatically notice each other. The DM determines who might be surprised.

The rules themselves describe surprise thusly:

Today we're going to dig deep into surprise, how the rules are intended to work, and some ways we can make it easier to run at the table. New to Sly Flourish? Start Here! Understanding Surprise in D&D 5eĪ few of 5th edition D&D's rules aren't quite as simple and clear as we'd like.

D&d 5e monsters by type