The temperature system is really good at ruining your day if you do not have a heat or cooling source around. You will notice that you get cold at night and during rain, while swimming, as well as while travelling in certain biomes. Likewise, you will also get too hot in certain biomes and conditions in which you are too close to sources of extreme heat (i.e. lava). In order to cool yourself down, you either need blocks of packed ice or to take a quick swim in a river or pond. To warm yourself up, have a campfire handy; it might just save your life.
This system serves to implement some added realism into the server, while also making survival harder (frankly, the way it should be). You may notice similarities to the mod called Tough As Nails. This is because this feature was inspired by it.