This means more bacteria was seen on the slide where they do the preliminary test. Now they will send the sample to be cultured and see what grows. This may indicate there was some contamination of the slide (not uncommon) or they got more or less actual sputum (as opposed to the intermixed saliva) on the slide, or the amount of mycobacteria in your lungs is not declining.
What comes back from the culture should lead the pulmonologist to the "next step" if you have been on the antibiotic for 6 months or longer.

Take a look at this video, at 23:00 minutes, for the latest in treatment choices for MAC that isn't improving after 6 months of antibiotics:

Now, for the hard part - can you relax, keep doing your airway clearance and exercise, and wait for the final word from the doctors?
Sue