When your dog is eating grass it usually means that they have an upset stomach and are trying to self induce vomiting by eating the grass.
A dog eating grass is perfectly fine and not harmful and it's a natural way for the dog to help an upset stomach because the grass will usually make the dog vomit.
Sometimes dogs will throw up after eating grass but not always and sometimes the dog may be eating grass improving digestion, treating intestinal worms, or fulfilling some unmet nutritional need, including the need for fiber.
There's nothing wrong with a dog eating grass as long as you've never put pesticides on that grass but eating grass is healthy for the dog and nothing to be worried about.