In this case, it's factoring in humidity into how hot it'll feel to you. A 90 degree day in bone-dry Phoenix feels a whole lot different than a 90 degree day in humid Atlanta.
Humidity affects the body's ability to cool itself off. When you sweat, the body is hoping that sweat will evaporate and cool you off. When the humidity is low, the air can accept water much more easily than when it's humid, so your sweat evaporates easier, and thus it'll feel cooler to you (Or, at least more comfortable.)
In humid areas, the air is already crowded with water droplets, making it more difficult for the sweat to evaporate and thus, you'll feel hotter (and sweatier.)
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