What is the true meaning for wording "POTATO" (40P GA inside) Close

First, my English skill is so bad, but I will try and hope you will understand. XD
I saw many thread talk about "POTATO" but I don't understand what is potato? potato as I know is just a food.
I pretty sure when I ask this question everyone will troll me, so I create this GA for my whitelist only!!
Everyone who tell me what is true meaning of "POTATO" will be added to my whitelist.

https://www.steamgifts.com/giveaway/sB66I/borderlands-the-pre-sequel
Close now, require Lv1+

What happen on someone who troll me???
May the POTATO mercy there soul.

Note. I will not reply any comment although someone will become my whitelist, so please check yourself.
Edit: This thread is not a joke I really want to know that.

Thank you for answer, now I understand about POTATO joke.
As I said above, this thread is not a joke so, you just give me a serious answer, I will add you on my white list.
But most people still troll me I don't know why.

May be most people in this thread are POTATO aren't ??