TL;DR is a card that lets you essentially double the subroutines on a piece of ice. It's kind of like getting two hamburger patties on your burger. Like if you wanted to go to Burger King or something and asked "Hey can I have a hamburger from you guys but with an extra patty on it?" and they said "OK." So you pay and wait for a little while. You wait, and wait, for what seems like an eternity. It must take a very long time to prepare another patty. After a while you get impatient and go up to the counter. The guy at the counter says "Hello, welcome to Jack in the Box may I take your order?" and you tell him "I already ordered. I ordered a hamburger with an extra patty on it." and he says "No you didn't." so you order again and pay again. See what I mean? It's like TL;DR making the runner do stuff again that they already did, and costing them more money.
TL;DR = TL;DR is like ordering a hamburger twice.