The main purpose of the base layer is to transport body moisture away from the skin.
Dry is warm. Water transmits heat
thirty times faster than air. Wet is cold. The base layer is worn next to the skin. Its purpose is provide moisture management by wicking away the moisture from your skin. Sweating is the body's way to cool down. This is great when it is hot and you are active & working hard but that same moisture can draw heat away from you leaving you cold & tired. Thus the need for a good wicking base layer to move that liquid away.
Good wicking fabrics are Non-absorbent synthetic fibers that wick moisture away from your skin.
Cotton is never a good choice for a base layer. Cotton clothing can absorb 30+% more
than its weight in water and transfers heat away from your body thirty
times quicker than dry air.
Don't confuse wicking with "breathability". Wicking clothes physically move liquid moisture away from your body whereas breathable outerwear allows moisture vapor to escape out through the fabric. Both work to keep you dry, but in different ways.
A good base layer will still need protection from the elements in the form of an outer layer. Depending on the conditions you may also need a good Mid-insulating layer to keep warm.