Activewear is sold on the ability to keep a wearer dry, and the mechanism is frequently described as breathability without further explanation. What the fabric actually does is move liquid water sideways.
Wicking is capillary action, not absorption
Narrow channels draw liquid along themselves, because the attraction between the liquid and the channel walls exceeds the liquid's internal cohesion.
A knitted structure of fine synthetic filaments creates thousands of such channels between the fibres, and sweat travels along them away from the skin.
The fibre itself absorbs almost nothing. The water sits in the spaces between fibres, which is why the fabric can be wet and still not feel saturated.
Spreading the moisture is what speeds drying
Evaporation happens at a surface, so the rate depends on how much surface area the water is spread across.
By pulling sweat outward and distributing it across the whole outer face of the fabric, a wicking knit dramatically increases that area.
The same volume of water on a small patch of cotton evaporates slowly, which is why a cotton shirt stays damp long after activity ends.
Cotton behaves in the opposite way
Cotton fibres absorb water into their structure, holding a substantial proportion of their own weight and swelling as they do.
Water held inside a fibre cannot evaporate until it migrates out, so drying is slow and the garment becomes heavy and clings.
That absorbency is a genuine advantage in a towel and a clear disadvantage in a garment worn during sustained effort in cold conditions.
Knit structure matters as much as fibre
The same polyester yarn can be knitted into a dense flat surface or an open mesh, and the two behave very differently.
Many performance fabrics use different yarn shapes or a double-layer knit, with a fine inner face against the skin and a more absorbent outer face to hold the transferred moisture.
Textured and grooved filament cross-sections increase the surface area within the yarn itself, improving capillary movement without changing the polymer.
Odour is the trade-off
Synthetic fibres are oleophilic, meaning they attract oils, and skin oils carry the compounds bacteria act on to produce odour.
Those oils bind to the fibre and are not fully removed by ordinary washing, which is why synthetic activewear retains smell that cotton does not.
Merino wool sits between the categories, moving moisture reasonably well while resisting odour, at higher cost and with more demanding care requirements.