Bestiality is a word describing sex between a human and an animal. There are few things stranger or more repulsive than bestiality.
When you say bestiality, you can hear the word beast, which is a good clue to the meaning. People who are into bestiality like having sex with animals. It doesn't matter what animal is involved: if someone if having sex with something alive that's not a person, it's bestiality. Bestiality is illegal and usually considered very wrong and harmful to everyone involved.