Lots of dogs eat grass. Sometimes they eat and swallow it; sometimes they seem to just grab and spit it out or chew and spit it out. No one knows for sure why they do it, but there are some theories.
It seems to be a commonly held belief that dogs eat grass when they feel nauseated. There was a study that looked into this and found that only 8% of plant eating dogs showed signs of illness prior to the grass eating and 22% vomited after eating grass. The dogs that seemed sick prior to the grass ingestion were the ones most likely to vomit after. Their study comprised more than 1500 client owned dogs.¹ So the numbers do not completely support that your dog eats grass because he feels ill, but many people would not agree with the findings and the dogs are not commenting.
Other people think that it is an indication of a dietary deficiency. The dogs in the study were fed a variety of different diets and there appeared to be no correlation between what the diet was comprised of and the likelihood of grass eating, so the explanation regarding dietary deficiency was not proven here either.1 Make sure that your dog eats a high-quality balanced diet so you do not have to worry about deficiency.