Temperature

Temperature is shown above the XP bar. The outer icon is the outside temperature, AKA how hot or cold it feels. The inner icon is the core temperature, AKA how warm you actually are.

If your core temperature is too hot or too cold, the icon will start to shake, and eventually you will take damage.

If you are too hot, you will get Hyperthermia , unless you have Heat Resistance.

If you are too cold, you will get Hypothermia , unless you have Cold Resistance.

There are numerous ways for temperature to change:

Altitude - How high up or below ground you are.
Armor - Special Armor heats you up or cools you down. There are also Enchantments.
Biome - Different biomes are naturally hotter or colder.
Blocks - Being near some blocks will heat you up or cool you down. A good example is the Campfire.
Season - If you have Serene Seasons installed, the season will have an effect.
Sprinting - Makes you warmer, so sprinting through a snow storm is a good idea.
Tiles - Some 'tile entities' change the temperature, like the Heater and Chiller
Weather - If you're in the rain or snow, it will cool you down.
Wet - If you're in a pool of water, you'll cool down as if you were in the rain.