i've been doing a lot of quests recently and i've received [item=layer card: overchest] and [item=layer card: foreground] but i have no idea what they actually do

They are involved in making "alchemy items" - however you need to have a Zombie skin to enter the place to use them. The zombie event happens in February, so until then, you won't actually be able to use them.

In the meantime, if you want to read up more about it, you can do so in this [topic not found]. Its an old thread that hasn't been updated in a long time, but the info is still valid.

If you have any further questions, feel free to ping me :).