Congratulations, Klains, on the great PSA test result!
Undetectable means that it is below the lowest value that the testing instrument can detect, for example, < 0.1, <0.05, <0.01, etc.
For almost all intents and purposes, anything less than 0.1 ng/mL is considered zero.
I had my PSA tested at 3 months, 6 months, 1 year, then yearly after that.