The three states of the Church. "When the Lord comes in glory, and all his angels with him, death will be no more and all things will be subject to him. But at the present time, some of his disciples are pilgrims on earth. Others have died and are being purified, while still others are in glory, contemplating 'in full light, God himself triune and one, exactly as he is"': All of us, however, in varying degrees and in different ways share in the same charity towards God and our neighbors, and we all sing the one hymn of glory to our God. All, indeed, who are of Christ and who have his Spirit form one Church and in Christ cleave together. CCC 954
This expression also refers to the communion between holy persons (sancti); that is, between those who by grace are united to the dead and risen Christ. Some are pilgrims on the earth; others, having passed from this life, are undergoing purification and are helped also by our prayers. Others already enjoy the glory of God and intercede for us. All of these together form in Christ one family, the Church, to the praise and glory of the Trinity. (#195)
This "great cloud of witnesses" (Heb 12:1) that is "closely united to Christ," (CCC 956) intercedes for us and the Church. In our weakness, the saints intercede for us and uphold the holiness of the Church. They are the Church Triumphant in glory with God in heaven, while we are the Church Militant struggling with sin and trying to be like Christ. There is also the Church Penitent, those who are saved, but being purified before entering glory. It is this last group that we can assist with our prayers. The Catechism Companion, Vol II notes:
The memory of saints and devotion to them contribute to the strengthening of the Church. Praying for the dead is a holy practice, through which we can help them... The saints' love for God and others is perfect in heaven, and by their love, our weakness is greatly helped. Christian communion among fellow believers brings us closer to Christ, and communion with saints joins us to Christ. (p. 26)
Finally, the Catechism Companion recommends this video with Fr. Mike Schmitz to clear up any confusion on Catholic veneration of saints: