“sad mortality o'ersways their power”
'O'ersways' means overpowers or overrules by superior force — death outweighs brass, stone, earth, and sea in any contest. 'Sad' here means grave or serious, not sorrowful.
“shall beauty hold a plea, Whose action is no stronger than a flower”
The poet casts beauty's struggle against time as a lawsuit it cannot win. 'Hold a plea' means sustain a legal case in court; 'action' is a legal term for a formal suit. Beauty's case collapses because its only evidence — a flower — will not last long enough for the hearing.
historical The sonnet form“wrackful siege of battering days, When rocks impregnable are not so stout, Nor gates of steel so strong but Time decays”
'Wrackful' means causing wreck and ruin — utterly destructive. The poet stacks military images: days become battering-rams, time lays siege, and even 'impregnable' rocks (those that cannot be captured by assault) and steel gates must eventually fall. 'Stout' means sturdy or resistant, not fat.
“Time's best jewel from Time's chest lie hid”
The poet imagines beauty as a jewel that belongs to Time — kept in Time's chest (treasure-box), from which there is no escape. The sestet's central question is whether anything can be hidden away from Time, which owns and ultimately claims everything beautiful.
“his spoil of beauty can forbid”
'Spoil' means plunder — what a raider carries off after sacking a city. Time is cast as a conqueror looting beauty; the question is who can 'forbid' (prevent) the robbery. The word carries the full weight of military conquest: beauty is left stripped and ruined.