MO stands for “mobile originated” message, whereas MT stands for “mobile terminated” message.
In the majority of cases, these terms describe messaging between individual cell phone users (e.g. consumers) and a platform (e.g. Marketron) from the platform’s perspective. MO denotes any message sent from a mobile phone to a platform and MT any message sent to a mobile phone from a platform.
For example, somebody sees a text call to action advertising “Text WIN to 44144”. They text in and receive a message back saying “You Win!” In this case, the original text in by the individual is an MO and the response “You Win!” message an MT.
Marketron only charges for MTs. Some providers charge for just MTs, others for both MTs and MOs. Charging for MTs is akin to a post office that charges consumers for a stamp to send each individual letter. Charging for both would be analogous to the post office requiring a stamp each time a person sent or received an envelope.