It's a sign of an upper respiratory infection.
Home treatment is to put her in the bathroom while you take a hot, steamy shower (okay, I've never seen results with this) or to give her some saline drops in her nose to help clean her nose out (GOOD LUCK I can only do it because this is our most docile cat).
If it continues, you should take her to the vet and get some antibiotics. In humans I'm strongly against antibiotics for sinus infections, but cats don't understand how to rinse/flush their sinuses so sometimes antibotics are the only thing that will work.
If it continues after a standard antibotic (or returns - Hunter has chronic sinus infections) then your vet can test to determine the strain of bacteria/virus/fungus causing the problem and you can try to re-treat based on that information.
Hunter's problems have continued long enough that there are actual visual changes in his lung x-rays (we thought for awhile that he had lungworm), and we're currently considering an immune suppressing medication to see if that helps. I'd encourage you to treat this seriously and early.