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. Under some circumstances, this may also include traffic that goes through an API to connect two people, however some countries (including the US and Canada) do not consider any application-mediated traffic to be P2P.
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.
Carriers in the United States and Canada have a strict interpretation of A2P, and consider all messaging that passes through Twilio (or other messaging application platforms) to be A2P.
Learn more about the difference between A2P and P2P SMS.