This is an extremely common issue. It means that one or more of your mods is malfunctioning, usually because of a conflict between two mods. This is why the order of how your mods load in Skyrim is important, as well.
Mod A adds NPCs, and homes them at Pinewatch. It mods Pinewatch in a way that makes sense for the mod.
Mod B replaces Pinewatch with a 12 story wizard tower. It appears lower in the load order.
Skyrim then loads A, and B, and when the assets for mod A are replaced by those from B during startup, the game crashes.
That's a really rudimentary explanation and a situation that doesn't always cause a crash, but you get the idea. Other things that can cause this include trying to load an Oldrim mod into Skyrim Special Edition, major game overhaul mods conflicting, or even a patch to Skyrim that breaks certain things.
To prevent this, I follow the following steps:
- Keep track of which mods do what and avoid situations where two mods accomplish the same basic task
- ALWAYS READ THE MOD DESCRIPTION FOR A CONFLICTS SECTION
- Use a mod manager like NMM
- Use LOOT to sort the mods for you. It will not only sort your load order, but it will notify you of detected conflicts before you try to run anything.
- Install your mods only 4 or 5 at a time, then see if you can load the game. that way, if you get a crash, you know which group of 4-5 mods to investigate.
- Make sure the mods you have also have any compatibility patches made for them to interface with other mods installed.
When it's already happened and I'm crashing on boot or on entry to an area, I do the following.
- Ensure all mods installed are using their newest versions
- Untick all of my mods in my mod manager and boot Skyrim. if it works, I tick 5 mods at a time and keep booting that way until I find the one that is causing the issue
- Look for new compatibility patches on the affected mod
- If all else fails, uninstall the mod completely instead of just disabling it.
Welcome to modding Bethesda games.