Mind you, I was not saying whether or not I agree or disagree with either the "ordinary infallibility of the Magisterium" or the "only ex cathedra statements are infallible" position - just pointing out that the two schools of thought do exist.
For those who aren't familiar with it, the "ordinary infallibility" concept basically means that if the Pope (or one of his duly authorized officers*) gives an instruction or a judgment, it is for all intents and purposes "infallible" in the sense that, unless it's a direct command to commit a sin, the faithful are bound to assent to it.
Basically it's a glorified version of the "chain of command", found in most large organizations (like the military) - if you belong to/work for the organization, you follow its rules and agree with its public statements. (If you don't, then you might want to reconsider your membership/employment.)
An "ex cathedra" pronouncement is super-serious, and not used very often, because once in place it *cannot* be reversed.(Please note, these are my impressions - my opinions - of what the two positions are - I'm neither a theologian nor a canon lawyer!)
(*Bishops, Priests, Nuns with/without rulers
, etc. )