What it means
Before loading, Forge and NeoForge check each mod has everything it needs. If not, the server stops with a list like:
Missing or unsupported mandatory dependencies:
Mod ID: 'geckolib', Requested by: 'alexsmobs', Expected range: '[4.4,)', Actual version: '[MISSING]'
Mod ID: 'neoforge', Requested by: 'create', Expected range: '[21.1.150,)', Actual version: '21.1.90'
Each line says which mod is missing or too old (Mod ID), who wants it (Requested by) and which versions will do (Expected range).
Reading the version range
[4.4,)means 4.4 or newer.[1.0,2.0)means at least 1.0 but below 2.0.[MISSING]means it isn't installed at all.
The fix
A mod is missing: install it from the Mods tab. Search the Mod ID (geckolib in the example) and pick the version for your Minecraft version and loader.
A mod is too old or too new: install a version inside the expected range.
The loader is too old (Mod ID forge, neoforge or minecraft): update Forge or NeoForge on the Startup tab to at least the version shown. If it asks for a different minecraft version, the mod is for another Minecraft version: replace it with the right build.
Restart and repeat until it starts. Installing one missing library sometimes reveals another.
Optional dependencies
Only mandatory dependencies stop the server. Lines about optional ones are just information.
Questions
What's a Mod ID?
The name a mod uses inside the game, usually close to its real name in lower case. Searching it on Modrinth or CurseForge finds the mod.
Does each player need these too?
If the mod adds content, yes. Players should use the same modpack as the server so everything matches.