A very simple story about 2 English soldiers that must deliver a message to a different company of soldiers to stop those soldiers from walking into a trap set by the Germans. You could compare it to "Saving Private Ryan"  because of the story line, but that wouldn't be fair. "Saving Private Ryan" is a much better movie.
"1917"'s biggest quality is the way it has been filmed. It looks like one long camera shot in real-time, for most of the movie's runtime anyway. This works surprisingly well for maybe half of the movie. Then it gets a little boring because of the weak story and a few clichés.
Who could have guessed there would be a friendly (and pretty) civilian and how many times can you naively trust an enemy soldier? There ain't really that many war scenes for a war movie, many of the soldiers are already dead and rotting when they meet them. Another problem is that having recently watched Peter Jackson's documentary "They Shall Not Grow Old"  it all looks way too clean and pretty. There was nothing pretty about the trenches in WW1.
The movie tries to deliver a message about how meaningless this war really was, but it will mostly be remembered for the impressive camera work.