In general, intercession is one’s praying or acting in behalf of another, usually at the request of the one to be benefited. The term is used in the Church in regard to the commendation of oneself to the saints.
The Council of Trent declared in the decree to the bishops concerning instruction that they teach the faithful, “the saints, ruling together with Christ, offer their prayers for men to God, and that it is good and useful to invoke them suppliantly and to have recourse to their prayers and to the powerful help in obtaining benefits from God through Jesus Christ.”
The saints follow Christ in interceding for us with God the Father (Rom. 8:34; Heb. 7:25). This act of veneration of the saints is performed not to obtain from the saints but through them. The saints, knowing of humanity’s needs and that Christ as Mediator bestows all favors of grace through Mary, His Mother, unite theirs with the prayers of Mary, direct them to Christ, and aid us as intermediaries.
This is an instance wherein, through intercession, the union of the Church militant and the Church triumphant, through Christ, acts for the salvation of mankind as well as for the individual members. (9:47)