Theologically, sacraments are different depending on the dogma of the church in question, but Luther was writing about Roman Catholic sacraments in particular. In the Roman Catholic belief, a sacrament represents a sacred ritual passed down by Jesus Christ to the church which represents a link to the divine salvation.

In the Roman Catholic Church, there are 7 holy sacraments: Baptism, the Eucharist (communion), Confirmation, penance, anointing of the ill, marriage and holy orders. In contrast, Luther's catechism limits the list of sacraments to just 2: Baptism and Eucharist. He explains that the rest of the sacraments, while important theologically, do not actually represent a tangible connection to God's promise of salvation. In the case of marriage, he maintains that it is primarily a union of a man and woman merely presided over by the grace of God. It does not represent a promise of eternal salvation in the way that the 2 Lutheran sacraments represent a joining in to the body of the faith.

It's important to understand the distinction in the teachings of Luther versus the Catholics. While Catholics maintained that faith AND righteous, good works were required for salvation and thus rituals related to a righteous lifestyle were considered sacraments, Luther argued that through faith ALONE is one saved. Therefore only the rituals related to the initiation to the faith are sacramental.

He does not exclude the other sacraments from general dogma (merely downgrading them from sacramental status), with the exception of holy orders which do not exist in the Lutheran Church.

Reference: Luther's Small Catechism or Apology of the Augsburg Confession

Vertex you might also want to X-post this in /r/Academicbiblical, /r/Theology & /r/ELINT

This old comment by /u/NotAVegetarian from the "What was the difference in the beliefs of Luther and Calvin?" thread might be helpful to you.