IPA: /ˈhɑːtər/
KK: /hɑtɚ/
Having a higher temperature than something else; warmer.
The soup is hotter than I expected.
Hotter → It is formed from "hot" (from Old English *hat*, meaning having a high temperature) and "-er" (a comparative suffix meaning more). The word "hotter" means having a higher temperature than something else.
Think of something that is 'more' ('-er') 'high in temperature' ('hot') — that's why hotter means having a higher temperature.