So does your post PSA result show < 0.1, the '<' is important because it means that they have tested to the limit that is possible in the lab. Your result being less than the threshold they can test to.
< 0.1 or 'less than' 0.1 would be considered undetectable.
If you need clarification of your result it's worth giving your uro-oncology nursed a call. The receptionist at my local GP are very good at missing out the all important '<'