This value is used by the model to constrain the list of possible species that it will try to detect, given the minimum occurence frequency. A 0.03 threshold means that for a species to be included in this list, it needs to, on average, be seen on at least 3% of historically submitted eBird checklists for your given lat/lon/current week of year. So, the lower the threshold, the rarer the species it will include.