apk – How does deodexing work?

As far as I understand, odexed apps come in two: the “apk” and the “odex” files, both containing different information. And I understand that the odex file has crucial information that loads first and speeds up the booting process.

But I tried to deodex an odexed app with UltimateDeodexer, and it only asked me to upload the .odex file, giving me in return an .apk file. Seemed like it turned the .odex into .apk.

How does that work?? Is it missing all the information from the original .apk file? Is my resulting .apk file complete, or am I supposed to do something else afterwards?