Fides ecclesiastica is a classification of those Roman Catholic dogmas which are Church teachings, definitively decided on by the Magisterium, but not (yet) as being Divine revelations properly speaking. They are considered infallible and irrevocable because, although they are not "truths of faith" (De Fide), they are nevertheless "closely related to them".
It is one level below De fide and one level above Sententia fidei proxima which are generally accepted as divine revelation but not defined as such by the Magisterium.
References
Fides ecclesiastica Wikipedia(Text) CC BY-SA