p100 is a rating given to respirators to indicate the amount of protection they are capable of providing. If a respirator has a p100 rating then it has the highest capabilities of filtering out whatever contaminants and particulates might be present in the air you are exposed to.
If your respirator has a p100 rating then you can use it around the liquid and solid aerosols that are considered to be harmful. This rating is required in places like petrochemical plants so that their workers are not exposed to harmful chemicals so basically this rating means the respirator is the best of the best.