- "å føle" is used to express how or what you feel. ("Jeg føler meg bra"; "Jeg føler sinne")
- "å føles" is used to express how something feels to you. ("Det føles godt"; "Den føles kald")
Note that "å føles" is not the passive form of "å føle", but a separate verb. It belongs to the s-verb category, where all the oddball verbs ending in -s in their active form end up.
Middle voice? Like, where instead of the Subject doing the Verb to the Object, the Subject and Object do the Verb to each other? Except sometimes it ends up being pretty idiomatic and you can't always just guess when to use it or how the voice will change the meaning of the verb.