PopHits (Population Hits or PH) refers to the number of genes in the background population (i.e. species full set of genes, genes on a microarray, etc) that are annotated to a specific term (i.e. p53 signaling pathway) in a specific category (i.e. KEGG). david.ncifcrf.gov/content.jsp?file=functional_annotation.html#fisher may be helpful for understanding these terms and how they are used for the statistics in DAVID.