Any mask should have at least 3 layers.
The outer layer is a mechanical filter to trap large particulates. It should be water resistant to repel large droplets.
The middle layer is an electrostatic filter to trap aerosols. It's manufactured using a process that imparts an electrostatic charge to the fibres.
The inner layer is a mechanical filter to trap aerosols and droplets produced by the wearer. It should be made of absorbent material.
The mask also needs to seal well around the face, including a stiff nose wire that should be shaped to fit the nose and seal against the cheeks. Many disposable masks don't have a stiff enough nose wire, and spread under the tension from the mask straps.
Ideally, the electrostatic filter should be removed before washing. Hot water and detergent damage the electrostatic properties, hence the recommendation to wash disposable masks in warm water without detergent, and no more than 10 times.