What it means? I always thought that the symbol way saying that the card will go and come to your head from discard/dead pile, but I read here in the forum "You can have more than tree copys of a card with the infinit symbol", it's true?

sorry about my english…

By itself, the infinity symbol means nothing. It doesn't give a card special powers or impose any limits. It's virtually flavor text in that respect.

Usually, cards with the infinity symbol have an ability that can be triggered from the discard or dead pile. But a card doesn't have to have the infinity symbol to have such an ability. (For example, Lady Stoneheart has an ability that can be triggered from the dead pile, but she does not have the infinity symbol.)

The infinity symbol is really just a reminder that appears on some cards that helps players stay aware of the fact that the card is there, in the dead or discard pile, to be used.

Thiagobhs said:

I'm sorry, but whoever told you that has no idea what they're talking about.

It's a useful tool if you stack your dead and discard piles so that only the title of each card is showing. This way you can see all the cards that are able to come back out. Unfortunately as Ktom pointed out, not every card has it…fail.