For sin shall not be master over you, for you are not under law but under grace. [Romans 6:14]
Suppose you used to work for company A, and now you are working for company B. Does the CEO of company A, your former employer, still have authority over you?
I guess this is the simplest example to explain the above verse.
The People’s New Testament explains that when the law is present together with our sinful nature, sin will repeatedly show up; but now we are under grace, and our sins were forgiven on the basis that we have died to sin, been buried and risen with Christ.