If a dog is pooping liquid with blood what does it mean?

It can mean many things, such as

-they ate wrong food for them

-Ate foreign object that upset stomach

-Raw hide toys that scratch the stomach

-Have a sickness that needs checked out by vet

think about what your puppy has eaten and if any of the habits have changed for instance, did u change their puppy food, is it puppy food or adult food, does it contain something in it that previous dog food didn't, The toys you give if a new one has been given is there anything missing that could harm them. And is the living area puppy friendly and he hasn't ate any hair ties or coins that can mess up his little tummy. If the diarrhea doesn't go away within 2 to 3 days take them to the vet. Check out their behavior as well. Are they sad and laying around, lazy, or aggressive.