Dismantling or Disassembling is the use of Science skill on most weapons, ammunition or armors to get various resources. When you dismantle an item, you have the chance to receive a portion of the items that are used to craft said weapon. This can't be done during combat.
You can dismantle only craftable stuff. There are some exception to this side you can't disassemble few kinds of ammunition like flamer ammo.
The higher your Science skill, the better chance you will have of getting more resources. The maximum amount of science for disassembling is 160%(there is no point of increasing since further than 160% won't increase your chances in dismantling.)
If you're science skill is low you can try to help yourself by using Tools or Super Tool Kit, the first one improve your science by 25%, the second one by 50%(Only for purposes of disassembling stuff, require keeping item in hand slot, both kinds of tools might break after using.)
The dismantled item condition is most probably affect how much you'll receive during process.
Note: You can disassemble items lying on the ground. Just press 7 and click on them. This is the most comfortable way of dealing with large amounts of useless stuff.
Starting from August 2015, Fonline2 players can mass disassemble all items. Nice, quick and easy! To mass disassemble all items on hex, stand over it and type ~decrafthex, to mass disassemble all items in your inventory type ~decraftinv. This will only work in bases that you can see or in your tents. You can also make a hotkeys for this actions by adding following lines in hotkeys.ini: decraftinv=hotkey and decrafthex=hotkey.