What is the cell reset?

[[JO] Janx]

I heard about that but what is it and when will it happen?

[Yhwach]

Cell Reset means that armors such as Power Armor will be reset to T-45, items in containers will revert back to whatever was inside them at default, enemies may or may not get reset but that's another reset.
It mainly happens when mods force a settlement or exterior cell to reset itself, this doesn't include things you placed down with the workshop, mainly containers and armors

[ironhead1969]

I have also heard of it happening when you use the scrapall console command.

[watchdog79]

This is a known bug that is part of the default game, but only gets triggered by any changes to world cells, such as those caused by settlement/player home mods.

If you are on PC, these two mods by lapdragon should help you. I personally tested them and they work like a charm:
http://www.nexusmods.com/fallout4/mods/6772/?
http://www.nexusmods.com/fallout4/mods/6698/?