RL means "Root Domains Linking". These metrics refer to the top 1-10 positions in Google and tells us from how many unique domains the backlinks are from.
RL Avg
Average number of root domain links (unique domains that link to the results) of Google places 1-10 for that keyword.
RL Max
Maximum number of root domain links (unique domains that link to the results) of Google places 1-10 for that keyword.