What does Max out your pet means?

thanks!

The training centre can only train pets up to a maximum of level 5000. So maxing out would be getting your pet to that level, so you can't train there anymore.

got it!

another hard goal.. thanks!

Actually, it's not quite right how it's worded. You get that achievement for hitting the stat cap, which is 22K total health, speed, strength, and defense.