Fit Test is a gauge of how comfortable and flush a mask is to the face. Comfort both reduces likelihood to touch the mask, thus increasing potential spread and extends how long a mask can be used before removing/overheating. The more flush a mask is to the face will reduce alternate airflow than through the air filter. Thus a mask must both be supple and utilize a low pressure drop (breathable) material.
The CDC provided this excellent infographic for describing exactly this importance of sealing surface.
The US Surgeon General Jerome Adams, in a tweet late February, asked Americans to stop shorting supply of face masks for health professions. Additionally the US Surgeon General cautioned on March 2nd that face masks must be used appropriately, as often face masks result in greater risk of infection due to heightened face touching. For this purpose, our face mask design focuses both on comfort and a snug fit to prevent necessary or accidental adjustments.