Hi, leading edge genes are those contributing to the significance of the enriched gene sets. They are genes ranked before where the peak of the enrichment score (a running sum) is reached. If you use ORA without scores, you can see genes annotated to a given gene set.
"""
The leading edge subset of a gene set is the subset of members that contribute most to the ES. For a positive ES (such as the one shown here), the leading edge subset is the set of members that appear in the ranked list prior to the peak score. For a negative ES, it is the set of members that appear subsequent to the peak score.