Base layers can often be an overlooked item, but really should be an essential part of any cycling kit.
They serve a couple of purposes.
Firstly to wick (remove; absorb) sweat off the body, which will help to keep you drier and cooler in the warmer months.
Secondly, in winter, a base layer will keep you warmer by adding an extra layer that helps to keep your body temperature warm while wicking any sweat which would cool you down and make you feel colder.
Choosing a baselayer.
When looking to buy a baselayer, consider the season you are buying for as well as the comfort. Look for low seams and tags that won't dig in. Keep in mind that you will have layers on top so it needs to be comfortable and not digging into your skin causing irritation.