Basically means your battery has fallen below a certain a certain percentage, and therefore won't last as long on battery compared to when it was new. However it is fine to keep using as is. If battery life worries you, you could get it replaced. What model is it? I think around 2009 they changed to internal batteries? My old macbook said it for a while, it doesn't matter TOO much. i replaced it after it said it for about 1.5 yrs but mine was an external battery so that was ez