This is the 4702nd most frequent Russian word.
жары
"Жары" in English means "heat" (plural form or context-dependent).
Here, 'жары' is the genitive form of 'жара', meaning heat, used to specify the undesired type of heat (intense).
Я не люблю сильной жары.
I do not like intense heat.
In this sentence, 'жары' is still in the genitive case, used to express abundance ('много' — much) of heat ('жары').
На улице было много жары этим летом.
There was a lot of heat outside this summer.
Here, 'жары' represents the cause of the decision ('жара' — heat, in the genitive case for expressing reason).