Tomatoes themselves are not toxic to dogs and dogs can safely eat tomatoes just like humans can eat healthy and delicious tomatoes.
However parts of the tomato plant such as the green parts and leaves of the tomato plants could be toxic to dogs so you should not allow the dog to eat any part of the tomato plant other than the tomato itself.
The leaves and other parts of the tomato plants contain solanine, a glycoalkoloid which is toxic if ingested by dogs and can lead to severe illness and possible death.
But the tomato itself is okay for dogs to eat and is healthy for them in moderation although if you give your dog too much tomatoes then it could cause stomach issues and possibly diarrhea.