When a dog’s tail is sticking straight up it can mean that they are confident, aroused, or excited. Some dogs display this behavior when they meet other dogs or animals. If their tail is upright and slightly shaking, it means that they are expecting a challenge.
When a dog is content, happy, or relaxed, the tail tends to be more level with the body. however, if their tail is low or between their legs it could mean they are insecure.