Day 21 progesterone levels! Help!!

So I'm new to this whole infertility thing... Been TTC for a year now and just began testing. My Husband was diagnosed with slow/sluggish swimmers and has motility issues. I am seemingly just fine. Yesterday I went for my 1st day 21 blood test and my levels were at 17.9! The nurse said that was great, but I don't know what that means! I am not on any type of fertility drugs and was 10 DPO. Can anyone explain to me what the levels mean and what that means for us!?