It is a type of material that stretches and is used especially for making clothes fit tightly.
Flexfit 6377 - Brushed 6-Panel Cap
Flexfit 6477 - Wooly Blend 6-Panel Cap
Flexfit 6577CD - Cool & Dry® Piqué Mesh Cap
Flexfit 180 - Delta Seamless Cap
Flexfit 6777 - Athletic Mesh Cap
New Balance MT61920 - Novelty Tech T-Shirt
Richardson 110 - Fitted Trucker with R-Flex Cap
Flexfit 6277Y - Youth Cotton Blend Cap