Paul uses the example of marriage, to illustrate and explain a relationship to the law through sin, versus our relationship to Christ through faith.
In this world, death dissolves all former relationships and contracts. That should seem simple enough.
The example of the marriage, when the husband dies, then the marriage is dissolved, and the wife is no longer bound by that marriage. That too, should seem simple enough.
But in this example, humanity is in the place of the wife,
the law of Moses is in the place of the former husband, and
Christ is in the place of the new husband.
Therefore, humanity is bound to the law through sin. This is the first marriage. And by the way, not one of our choosing.
When a person believes in Christ, that faith simultaneously identifies the person in the death of Christ, with respect to sin.
In that death, the individual is no longer bound by the constraints of sin. This prepares the way for a new marriage, or one of our choosing.
Just as a wife should not and certainly cannot, continue a relationship with a former and dead husband, so too a person who has believed in Christ should not attempt to revitalize a relationship with the former way of life, which was sin.
That former way of life, sin under the law of Moses, is of works, or an attempt to justify or advance oneself before God, by means of their own effort. But works is rejected and is not a part of the spiritual life, nor the relationship with God through Christ.
Therefore, through faith we have died to the former way, namely works through sin, and now are bound by grace through faith.