The question means 'What time do we board?', probably referring to a flight. The verb form here is just 'embarque', meaning to embark, or to board a boat or plane. The word 'on' means 'we', or people in general. For example, 'We board' is 'On embarque'. Because it's a question, we invert these two words, so that the verb comes first. The 't' between the two words has no meaning - it is just inserted to make the inverted verb and subject easier to pronounce. It is difficult to say 'embarque on', so we connect the two words with a 't' sound, and say 'embarque-t-on'. You'll sometimes see the letter 'l' ( L) used in the same way. Note that it should be 'heure', rather than 'heures'. I hope that helps.