P2P stands for Person-to-Person messaging. In simple terms, this is when two or more people communicate over text messaging. For example, when you send a text message from your phone to another person's phone, that's P2P. This can also include traffic that goes through an API to connect two people.
SMS is most often referred to as P2P when compared with A2P (Application-to-person) messaging. Wireless carriers treat P2P and A2P traffic differently in terms of the type and volume of traffic allowed. To avoid carrier filtering it is important to understand your use case so you can use the correct tools such as phone numbers, short codes, and routing intelligence.
Learn more about the difference between A2P and P2P SMS.