This sonnet ponders the speakers eventual death and tries to comfort his lover. Let's break it down and see if the meaning can be more clearly realized.
Lines 1-2: "But be contented when that fell arrest/With bail shall carry me away." "Fell arrest" is fearful death; "without bail" means that there is no deal to be made with mortality. However, the speaker begins the lines with "Be contented," so what is to come should be reassuring.
Lines 3-4: "My life hath in this line some interest,/Which for memorial still with thee hall stay." Here "line" is the verse he is writing, the "interest" is the "legal claim" that he had once lived and loved and the sonnet will be with her always, even if he is not physically there.
Lines 5-6: "When thou reviewest this, thou dost review/The very part of was consecrate to thee." When you read this, you read the part of me that is immortal. It was written for you, he says, and will belong to you always.
Lines 7-8: "The earth can have the earth, which is his due;/My spirit is thine, the better part of me." We cannot escape death, but death has no power over love eternal.
Lines 9-12: "So then thou hast lost the dregs of life,The prey of worms, my body being dead,/The coward conquest of a wretch's knife,/Too base of thee to be remembered." The cowardly conquest of a wretch is Death. But it is not his physical body that the lover should remember,it is his love for her.
Lines 13-14: The worth of that is that which it contains/And that is this, and this with thee remains." Again, it is not the corporeal body that endures, but the love expressed in the verse.