Grass is safe for dogs to eat as long as the grass has no fertilizer on it.
If you take your dog to a park and are not sure if the grass has any fertilizer or other chemicals on it then you shouldn't let your dog eat the grass because the fertilizer or chemicals can be bad for the dog and possibly kill the dog.
But if the dog is eating grass that is not fertilized such as at your own house then the grass is safe for the dog to eat.
Dogs eat grass usually to try to make themselves throw up or to settle their stomach when they are having stomach issues such as an upset stomach.
My dog eats grass and every time he throws up a few hours later.
He usually has an upset stomach when he eats the grass.
The dog might also be eating the grass to get nutrients from the grass.
Grass is healthy for the dog and there's no harm in them eating the grass.