What does it mean for a dog to go into heat?
It means she's going to bleed like a girl would during her period, and she's receptive to males. If you don't want baby puppies running around, you should keep her as far away from non-neutered males as possible. Male dogs will do ANYTHING to get to a girl dog in heat.
it means it is the time when your dog is ready to mate and get pregnant, so unless you do not want any puppies then keep her well away from any male dogs, until her season is finished.
and it means they will get really horny and bleed out of their.. well you know what I mean.
GOOD LUCK = ]
It means she's ready to mate and have babies. =]