Delves are soloable mini-dungeons. They aren’t mechanically complex (because they need to be doable by every class/spec) but things hit hard. This results in a less fast-paced, more methodical experience, having similar vibes to the older BC-Cata dungeons. A benefit of delves is that they aren’t timed so you can take a break whenever you want.
Their reward structure is the opposite of m+. M+ is very rng but infinitely farmable. Delve rewards are guaranteed but have weekly limits. M+ gears up in less calendar time while delves gear up with less time played.
The best gear from delves is obtained via crafting, making it even more deterministic. It would take half a season but a delve only player could gear up to a mid-mythic raid level if they were of a mind to do so. There are also cosmetics, mounts and transmogs unique to delves.