Micro fiber
Very fine polypropylene or nylon fibers dosed into concrete primarily to reduce plastic shrinkage cracking during the first hours after placement.
What does micro fiber mean?
Micro fibers work while the concrete is still plastic, distributing early tensile stress before the material has any strength of its own. They provide essentially no structural benefit after hardening and do not substitute for mesh, rebar, or macro fibers. Because they are hair-fine, they can leave whiskers on the finished surface that burn off or wear away.
In practice
They are cheap, frequently included by default in residential mixes, and often mistakenly sold to homeowners as a replacement for reinforcement.