If something is middling, it isn't terrible and it isn't great — it's mediocre. You might describe your chances of beating your dad at tennis as middling.
You'll frequently find this adjective in the phrase "fair to middling," or sometimes "good to middling." This American colloquialism has been used since the early 17th century to mean "slightly above average," and it originally referred to the quality of farm goods. Today you can use it to rate or grade anything, so you might say, "I thought this peach would be juicy and perfectly ripe, but it's fair to middling."