Sorry freewheeler! "OP" is meant as an abbreviation for "Original Poster", meaning in this case YOU, the person who started this thread.
As for the matter at hand, Barit1 has already explained the principle more clearly and succinctly than I could hope to do. Just know that high temperature with a high dewpoint means that the air density is even less than accounted for by considering the effect of temperature alone. High, hot and humid is one condition where published takeoff performance data might best be used with a little extra conservatism.
Best,
westhawk