TelAPI numbers can be configured to respond to calls or SMS messages with InboundXML. This page explains that a bit further.
When TelAPI receives a InboundXML document, the "instructions" are contained within the <Response> </Response> elements. TelAPI begins reading at <Response> and behaves accordingly as it encounters each new instruction element until the end of the </Response>. All InboundXML elements are camelCased and are categorized as either Verbs or Nouns.
InboundXML verbs are elements that define the behavior the SMS should have when executing the InboundXML. Nouns are simply the specifics of the behavior defined by the verb elements. Nouns are always nested within verb elements and can be either other xml elements or plain text.