The supine is a verbal noun. In ordinary verbs it is formed by adding -tum to the present stem, with the modification of e to i in the second conjugation (mone- > moni-)
When a supine is translated to English, the suffix -tum turns into -tion. Here are some examples.
sedeo I sit sessum session informo I inform informatum information traho I pull tractum traction moveo I move motum motion dico I speak dictum diction
There are lots of words in Latin that end in -um; the supine is just one of them.