The mysteries of the church are certain rituals that are not only imbued with symbolic meaning, but also have metaphysical, spiritual significance. But we do not know what that is! Thus, we call them “mysteries” or sacraments (from the word “sacred” meaning really special).
The seven mysteries are:
1. Baptism
2. Chrismation (anointing of oil on infant)
3. Communion
4. Confession
5. Marriage
6. Holy Unction (anointing of oil)
7. Ordination
In each of these rituals, we believe something happens metaphysically, that is, beyond the physical realm, in the spiritual realm. We do not exactly know what that is, but just trust that it is good and beneficial for us - thus we call it a “mystery”.