When something is only okay or mediocre, it's so-so. If that new action movie had a few exciting scenes but an implausible plot and some uneven acting, you might say it was just so-so.
So-so is perfect for describing things that fall right in between terrible and spectacular. A basketball season in which your team wins half the games and loses the other half is a so-so season. And a movie book that you didn't hate but wouldn't recommend to all of your friends is also so-so. In French you'd say "Comme çi comme ça," or "like this, like that," and in Swahili you might say "Nusu nusu," or "half and half."