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If you mean what I think you mean, it's when the internal battery in the game dies. This renders your game litterally unusable as you can't save it. You can send it to Nintendo to get it fixed, but you might as well go out and buy a new one somewhere. You can usually get the game for about $10 I believe.
Is the battery inside the cartridge? And how long does it last?
Yes, and the longevity of it varies. I got my game soon after it came out and it's still working fine. There's been some cases (don't know exactly how many people it's happened to) of the internal battery dying a few years after purchase. It's basically random as to when/if it dies.