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.